Grease a 9x13-inch baking dish with butter. Cut the French bread into bite-sized cubes, using slightly stale bread if possible since it absorbs the custard better. In a skillet over medium heat, cook the breakfast sausage until browned, breaking it into small crumbles. Add the diced onion and cook until softened, then stir in the spinach and cook until just wilted. Remove from the heat and let the mixture cool slightly.

Step 2: Assemble the Strata

Spread the bread cubes evenly in the prepared baking dish. Scatter the cooked sausage, onion, and spinach mixture over the bread, making sure everything is evenly distributed. Sprinkle the shredded cheddar and Gruyère cheeses over the top to create a rich, cheesy layer throughout the casserole.

Step 3: Make the Custard

In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, Dijon mustard, salt, and black pepper until the mixture is completely smooth. Slowly pour the custard evenly over the bread and fillings, ensuring every piece of bread is coated. Gently press the bread down with the back of a spoon so it can fully absorb the liquid.

Step 4: Chill Overnight

Cover the baking dish tightly with plastic wrap or foil and refrigerate for at least 8 hours or overnight. This resting time allows the bread to soak up the custard, giving the Ina Garten Breakfast Strata Recipe its creamy texture and rich flavor.

Step 5: Bake and Serve

Preheat your oven to 350°F (175°C). Bake the strata for 45–50 minutes, or until the top is golden brown and the center is fully set. Let it rest for 10 minutes before slicing. Serve warm and enjoy this delicious, cheesy, and comforting Ina Garten Breakfast Strata Recipe.