Heat your oven to 275 degrees. Pour enough tomato sauce into a casserole to coat the bottom of the dish. Add half of the mushrooms and spinach on top.

Then generously dust the chicken with salt and pepper. Heat a skillet over medium-high heat, and melt half of the butter in the skillet. Add the chicken, being careful not to overcrowd the pan. Cook each side of the chicken for about 2 minutes, until brown. Add the remaining butter as necessary. Place the chicken in the casserole dish.

Pour the rest of the vegetables over the chicken. Then pour enough tomato sauce over the chicken and vegetables to cover them. Top with slices of mozzarella.

Bake for 1 hour, and then finish by turning up the heat to broil (make sure your dish can handle this) just to brown the cheese.

Serve with pasta.