Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking dish.

Cut the stale bread into bite-sized cubes and layer them at the bottom of the greased baking dish.

In a large skillet, cook the breakfast sausage over medium heat until golden brown, about 5-7 minutes. Then, add mushrooms and spinach, cooking until softened, about 3-4 minutes.

In a mixing bowl, whisk together the eggs, milk, and sharp cheddar cheese, adding salt and pepper to taste.

Fold the sausage and vegetable mixture into the egg mixture and pour it evenly over the bread layers in the dish.

Bake in the preheated oven for about 50 minutes, until the top is golden brown and a knife comes out clean.

Remove from the oven and let it cool for 10-15 minutes before slicing.