Melt butter in a 10-inch, oven-safe skillet. Sautee onions, peppers, smoked sausage and minced garlic. Turn off heat and set aside. In a separate bowl, whisk eggs, add parmesan cheese, Worcestershire sauce, salt and pepper. Pour egg mixture in with sautéed onions, peppers and sausage. Mix. Evenly distribute tomatoes and fresh spinach on top of egg mixture in skillet. Use a fork to press spinach and tomatoes slightly down into eggs, and then top with (optional) mushrooms. Carefully place skillet in the oven and bake for 15-20 minutes. Test with fork to ensure eggs are completely cooked before removing from oven. Let frittata rest 5-10 minutes before cutting into pie slices.