Heat a skillet over medium heat; cook and stir sausage until crumbly and completely browned, about 10 minutes. Transfer cooked sausage to the prepared casserole dish.
Cook and stir mushrooms in the same skillet over medium heat until liquid has been released and mushrooms are lightly browned, 5 to 10 minutes; drain.
Mix mushrooms, eggs, bread, milk, Cheddar cheese, ham, spinach, flour, mustard powder, salt, butter, and basil together in a large bowl; pour over sausage. Cover casserole dish and refrigerate, 2 hours to overnight.
Preheat oven to 350 degrees F (175 degrees C).
Bake in the preheated oven until a knife inserted into the center of the strata comes out clean, 60 to 70 minutes.