Start by heating a large skillet over medium heat. Add 2 tablespoons unsalted butter and allow it to melt completely. This will be the foundation for sautéing your vegetables.
Once the butter is melted, toss in the 8 ounces sliced mushrooms. Sauté them for about 3 to 4 minutes, stirring occasionally. You'll notice them beginning to brown and caramelize, releasing their wonderful flavor.
Next, add 10 ounces thawed and drained frozen spinach to the skillet. Sauté briefly until it wilts, which should take another few minutes. The mixture will become vibrant, and the spinach will blend beautifully with the mushrooms.
Remove the skillet from the heat and transfer the mushroom and spinach mixture to a large mixing bowl. Let it cool slightly while you prepare the other ingredients.
In the same skillet, add the cooked 1 pound crumbled breakfast sausage and the diced 8 cups crusty bread to the mushroom and spinach. This combination is where the magic begins, as the bread will soak up all the flavors.
In a separate bowl, whisk together 12 large eggs, 2 cups whole milk, ½ teaspoon thyme, ¼ teaspoon garlic powder, ½ teaspoon onion powder, 1 ½ teaspoon salt, ½ teaspoon ground pepper, and 2 tablespoons dijon. This custard mixture is what will bind everything and give you that creamy texture.
Whisk in 1 ½ cups shredded cheddar cheese until well combined. Pour this mixture over the bread and sausage mixture in the bowl, tossing gently until everything is nicely coated.
Cover the bowl with plastic wrap and refrigerate for 30 minutes to an hour. This allows the bread to soak up the custard, ensuring a moist and flavorful strata.
Preheat your oven to 350 degrees Fahrenheit. Select a 9×13 or a 4-quart baking dish and spray it lightly with pan spray. This will prevent sticking and ensure easy serving.
After the chilling time, take the strata mixture from the fridge and pour it into the prepared baking dish. Spread it out evenly.
Place the dish in the oven and bake for 45 to 50 minutes. You’ll want the top to be golden brown and the center to be set with no jiggle.
Once baked, let the strata cool for 5 to 10 minutes before serving. This cooling time helps it firm up, making it easier to slice and serve.