Heat a large skillet over medium/high heat. Add the breakfast sausage and cook until partially done. Remove the sausage from the pan, leaving the drippings.
Saute the onion in the sausage drippings until soft. Add mushrooms, olive oil, and cook for 5 more minutes. Then, add garlic and spinach, cooking until wilted.
Whisk eggs, heavy cream, and milk in a bowl.
Spray a 9×13-inch casserole dish and add ripped bread pieces. Add cooked veggies, sausage, 1 cup cheddar cheese, 1 cup gruyere cheese, salt, pepper, and toss.
Pour egg mixture over the ingredients and press down gently.
Refrigerate the strata overnight.
Preheat oven to 350ºF, toss strata, and bake covered for 30 minutes. Uncover and bake for another 30 minutes.
Add remaining cheese on top and bake until golden brown.
Let it rest for 10 minutes before slicing.