In a large, oiled skillet, saute sausage, onion, and mushrooms over medium-high heat, stirring and breaking up sausage, until meat is browned.
Spread prepared dough over the bottom and up the sides of a greased, 12-inch cast iron skillet. Sprinkle one cup of mozzarella over the uncooked crust. Top with the sausage mixture. Then spoon the pizza sauce over the top, followed by the second cup of mozzarella. Dot the surface with small pieces of fontina, then sprinkle with the thyme.
Bake at 400 degrees for 20-25 minutes or until crust is cooked through and the top is bubbly. Allow pizza to stand for 5-10 minutes before serving.