Frittatas can be cooked on top of the stove or, often, are started on top of the stove and finished in the oven. Cast iron pans are often used as they can go from stovetop to oven seamlessly.    For our version, we wanted to provide instant portion control, so ours are cooked in muffin tins, but use classic frittata fillings. They also freeze very well in these small sizes.