MAKE AHEAD:
The mini frittatas will keep in an airtight container in the refrigerator for 3 to 5 days. When hunger strikes, place 1 or 2 frittatas on a microwave-safe plate and microwave on high power for 30 to 40 seconds, or until heated through and steaming. (Keep in mind that my microwave is quite old and has less power than most, so your microwave time will likely vary.) Alternatively, you can reheat the mini frittatas in a warm oven or a toaster oven.
GET CREATIVE:
There are really no hard-and-fast rules for what constitutes an “acceptable” filling for these mini frittatas. Let your palate be your guide: If you think the ingredients would taste good with eggs (especially if you’ve previously enjoyed a particular flavor combination in an omelet or a quiche), then they’ll probably make a yummy filling for your frittatas.
You can substitute 2 cups of your favorite vegetable(s), chopped into small, bite-sized pieces, for the vegetables in the main recipe above. Depending on the veggies you choose, you might not need to pre-cook them before adding them to the egg mixture. For example, spinach and other greens don’t release a lot of water as they bake in these frittatas, so they don’t need to be pre-cooked. Other vegetables, however, might release too much moisture and make the frittatas watery (e.g., mushrooms, zucchini) and/or won’t be fully cooked through by the time the frittatas are done (e.g., potatoes). For these veggies, it’s best to cook them first before adding them to the egg mixture (a quick sauté in a little olive oil should do the trick). You can also use leftover cooked vegetables, regardless of how they were cooked—sautéed, roasted, or steamed. Just be sure the veggies are at room temperature (cooled after cooking or brought to room temperature after being in the fridge) before adding them to the egg mixture. And if you have any particularly vegetable-averse kiddos (or adults) in your household, you might find that finely chopping the vegetables in a food processor so that they nearly disappear into the frittatas can help make pickier palates more willing to try them.
As for the cheese, you can substitute ½ cup of your favorite cheese (shredded, grated, or crumbled) for the cheese in the recipe above. You can also swap out the ¼ cup of grated Parmesan cheese in the base recipe for an additional ¼ cup of the cheese of your choice. I like the extra layer of flavor that the Parmesan adds to the frittatas, but it’s totally your call.
My carnivorous boys would also like to point out that cooked, crumbled bacon or cooked, diced ham are welcome additions to mini frittatas.
And if you want to “dress up” these frittatas or add another boost of flavor, you can serve them with a dollop of salsa or pesto, a few slices of avocado, or a sprinkle of fresh herbs.
One last tip: The base recipe halves neatly, so if you’d like, you can make 2 different flavors of frittatas in a single muffin tin. Just mix up 2 half batches of the egg base (4 eggs + ¼ cup milk + 2 tablespoons Parmesan cheese) in 2 separate bowls, adding a different filling (1 cup veggies + ¼ cup cheese) to each, then divide each egg mixture evenly among 6 muffin cups.