Quick Recipe Version (TL;DR)
Quick Ingredients
- 12 oz (about 8 cups) sturdy bread cubes (3/4-inch)
- 3 tbsp unsalted butter + 1 tbsp olive oil
- 2 large yellow onions (about 1 lb), thinly sliced
- 1/2 tsp kosher salt (for onions) + 1 tsp kosher salt (for custard)
- 6 oz baby spinach
- 8 oz Gruyère, grated (about 2 cups)
- 10 large eggs
- 2 1/2 cups whole milk
- 1 cup heavy cream
- 1 tsp Dijon mustard
- 1/2 tsp freshly ground black pepper
- 1/4 tsp ground nutmeg
- 2 tbsp chopped chives or parsley (optional)
Do This
- 1) Caramelize onions in butter + oil over medium-low, 35–45 minutes; cool slightly.
- 2) Wilt spinach 1–2 minutes; squeeze dry and chop.
- 3) Butter a 9×13-inch baking dish; add bread cubes, onions, spinach, and Gruyère.
- 4) Whisk eggs, milk, cream, Dijon, salt, pepper, and nutmeg; pour over.
- 5) Press gently so bread soaks; cover and refrigerate 10–12 hours.
- 6) Bake at 350°F (175°C) covered 30 minutes, then uncovered 25–35 minutes until puffed and 160°F in the center.
- 7) Rest 15 minutes, garnish with chives, slice, and serve.
Why You’ll Love This Recipe
- Perfect for brunch: Make it the night before, bake in the morning, and serve a crowd with minimal fuss.
- Big flavor, simple ingredients: Sweet caramelized onions, earthy spinach, and nutty Gruyère make every bite feel special.
- Sliceable and reliable: The custardy center and browned top hold together beautifully after a short rest.
- Flexible: Easy to swap greens, cheeses, or add a breakfast protein if you’d like.
Grocery List
- Produce: 2 large yellow onions, 6 oz baby spinach, chives or parsley (optional)
- Dairy: unsalted butter, whole milk, heavy cream, 8 oz Gruyère
- Pantry: sturdy bread (sourdough/French), olive oil, Dijon mustard, kosher salt, black pepper, ground nutmeg
Full Ingredients
For the strata base
- Bread: 12 oz sturdy bread (about 8 cups 3/4-inch cubes), preferably day-old (sourdough, French, or country loaf)
- Gruyère: 8 oz Gruyère, grated (about 2 cups), divided
- Optional garnish: 2 tbsp finely chopped chives or parsley
For the caramelized onions
- 2 large yellow onions (about 1 lb), halved and thinly sliced
- 3 tbsp unsalted butter
- 1 tbsp olive oil
- 1/2 tsp kosher salt
For the spinach
- 6 oz baby spinach
For the savory egg custard
- 10 large eggs
- 2 1/2 cups whole milk
- 1 cup heavy cream
- 1 tsp Dijon mustard
- 1 tsp kosher salt
- 1/2 tsp freshly ground black pepper
- 1/4 tsp ground nutmeg
Step-by-Step Instructions
Step 1: Prep the bread and pan
Cut the bread into 3/4-inch cubes (you want bite-size pieces that still have structure). If the bread feels very fresh or soft, spread the cubes on a sheet pan and bake at 300°F (150°C) for 10–15 minutes to dry them slightly, then cool. This helps the strata soak up custard without turning gummy.
Lightly butter a 9×13-inch baking dish (or coat with nonstick spray if you prefer). Set aside.
Step 2: Caramelize the onions
In a large skillet over medium-low heat, melt 3 tbsp unsalted butter with 1 tbsp olive oil. Add the sliced onions and 1/2 tsp kosher salt and stir to coat.
Cook, stirring every 3–5 minutes, until the onions are deeply golden and sweet, 35–45 minutes. If they begin to stick or brown too quickly, lower the heat and add 1–2 tbsp water to loosen the browned bits.
When done, spread the onions on a plate to cool for 5–10 minutes. (Cooling slightly prevents the eggs from starting to set during assembly.)
Step 3: Wilt and drain the spinach
Using the same skillet (no need to wash), add the spinach and cook over medium heat until just wilted, 1–2 minutes, stirring constantly.
Transfer spinach to a colander and press firmly with the back of a spoon to remove excess moisture. Once cool enough to handle, squeeze once more with your hands, then roughly chop. Removing water is key so your strata bakes up rich, not watery.
Step 4: Whisk the custard
In a large bowl, whisk together 10 eggs, 2 1/2 cups whole milk, 1 cup heavy cream, 1 tsp Dijon mustard, 1 tsp kosher salt, 1/2 tsp black pepper, and 1/4 tsp nutmeg until very smooth and evenly combined.
Step 5: Layer and soak overnight
Add half the bread cubes to the baking dish. Scatter on half the caramelized onions, half the spinach, and about half the Gruyère. Repeat with the remaining bread, onions, spinach, and reserve about 1/2 cup Gruyère for the top.
Pour the custard evenly over the strata. Use your hands or a spatula to press down gently so the bread is in contact with the liquid. Sprinkle the reserved Gruyère over the top.
Cover tightly with foil and refrigerate for 10–12 hours (overnight).
Step 6: Bake until puffed and set
In the morning, let the strata sit at room temperature for 20 minutes while you preheat the oven to 350°F (175°C).
Bake covered with foil for 30 minutes. Remove the foil and continue baking until puffed, nicely browned on top, and the center registers 160°F (71°C) on an instant-read thermometer, 25–35 minutes more.
If the top is browning faster than the center is setting, tent loosely with foil for the last 10–15 minutes.
Step 7: Rest, slice, and serve
Let the strata rest for 15 minutes before slicing. This helps it set up so you get clean, sliceable portions rather than a scoopable casserole.
Finish with 2 tbsp chopped chives or parsley if using. Slice into squares and serve warm.
Pro Tips
- Don’t rush the onions: True caramelization takes time. Medium-low heat builds sweetness and deep flavor without burning.
- Dry bread = better texture: Slightly stale or briefly oven-dried bread gives a custardy interior with structure, not mush.
- Drain the spinach well: Press and squeeze out moisture so the custard sets properly.
- Use a thermometer: Pull the strata when the center hits 160°F (71°C) for a set, tender custard.
- Rest before cutting: A 15-minute rest makes it far easier to serve neat slices.
Variations
- Add a breakfast protein: Layer in 8 oz cooked breakfast sausage or 8 slices cooked bacon (crumbled) with the onions.
- Swap the greens: Use 6 oz chopped kale (sauté 3–4 minutes until tender) or 6 oz Swiss chard (stems sautéed first).
- Change the cheese: Replace half the Gruyère with sharp white cheddar for a more assertive, tangy bite (keep total cheese at 8 oz).
Storage & Make-Ahead
Make-ahead: This recipe is designed to be assembled and soaked 10–12 hours ahead. You can also caramelize the onions up to 3 days in advance; refrigerate in an airtight container and bring to room temperature before layering.
Refrigerate leftovers: Cool completely, then cover and refrigerate for up to 4 days.
Reheat: Warm individual slices in a 350°F (175°C) oven for 10–15 minutes, or microwave gently (the oven keeps the best texture).
Freeze: Wrap slices tightly and freeze up to 2 months. Thaw overnight in the fridge and reheat at 350°F (175°C) until hot throughout, 15–20 minutes.
Nutrition (per serving)
Approximate, based on 10 servings: 420 calories, 22 g protein, 28 g carbohydrates, 25 g fat, 2 g fiber, 6 g sugar, 650 mg sodium.
