Fry the sausage in a large skillet over medium-high heat, breaking it up as it cooks into small pieces with a spatula. When the sausage is fully cooked, drain off any excess liquid, remove the meat from the skillet with a slotted spoon or spatula, and place on a paper towel to drain.

In the skillet over medium-high heat, melt the butter. Add the chopped onion and mushrooms, sautéing them until the onion is translucent and the mushrooms are softened. Add in the spinach, stirring every couple of minutes until the spinach has fully wilted. Remove from heat and combine with the cooked sausage in a medium bowl. In a separate large bowl, whisk together the eggs, milk, herbes de Provence, salt, and pepper.

Using a 9×13-inch baking dish (or other dish of comparable size), alternate layers of bread cubes, sausage and vegetable mixture, and cheese, repeating until the dish is filled. Make sure that cheese is the top layer. Pour the seasoned egg and milk mixture evenly over the strata so that the liquid fills the gaps in the dish.

Let rest for at least 1 hour before baking. If preparing the dish the night before, refrigerate the strata overnight then let it sit out for 30 minutes so it isn’t cold when it goes in the oven.

Preheat the oven to 350°F. Bake the strata, covered, for 40 minutes or until it has set well. Remove cover, then turn on the broiler, and broil until the top has browned – just a few minutes. Watch it closely so the cheese doesn’t burn. Remove from the oven and let rest for about 15 minutes before serving.