1. Preheat oven to 350°F.
2. In a cast-iron skillet, heat the butter and oil. Add the optional garlic and the sliced mushrooms and fennel and cook over low heat until they are very soft, about 10 minutes. If you are using uncooked tomatoes and herbs, add them in after about 5 minutes. If you are using defrosted cooked tomatoes (or fresh chopped tomatoes), just mix them in at the end.
3. Crack the eggs into a large bowl. Add the ricotta and whisk well. Pour egg mixture over the vegetables.
4. If using a cast-iron skillet, you can use it to bake your frittata. If you are not using cast iron, or if you don't want to bake your fennel frittata right in the skillet, you can butter or oil a deep pie dish or other casserole pan and pour in the frittata ingredients.
5. Bake for 15-20 minutes or until the frittata is set. Allow to cool before slicing.