Combine the tomato sauce, bone broth or water, spices and salt in a saucepan and simmer for 10 mins. Add the carnitas and sliced onion and simmer an additional 5 mins. Pour the mixture into a small casserole – equivalent to a 6- or 8-inch round or square. Top with sliced mozzarella and bake for 30 minutes.
Remove from oven and let rest 10 mins. Top with cotija and cilantro.If you are looking to reduce calories and fat, one or both of the cheeses can be removed from this recipe without modifying the technique.