Preheat oven to 375°F | 190°C.I sometimes turn the oven up to 400° for a shorter cook time.
In a large, oven-safe skillet, sauté vegetables and protein in olive oil over medium heat until soft. I use a 10- or 12-inch cast iron skillet, depending on how big a crowd I’m feeding.You want between 2 and 3 cups of cooked veggies and protein when everything is done.
In a medium bowl, whisk together 6 eggs & ½ milk. Stir in herbs and spices.I sometimes use 1 or 2 more eggs, especially when using a 12-inch skillet.
Pour eggs evenly over skillet mixture. The egg mixture should just barely reach the top of the veggie mix in skillet. Once you pour in the egg mixture, stir it in a little so that it settles in and around the ingredients in the pan. The egg mixture should just barely reach the top of the veggie mix in skillet. If it looks like there aren’t enough eggs, go ahead and whisk up another and pour it in. Don’t go crazy though – remember, the egg mixture will raise up quite a bit while baking.Sprinkle cheese over top and shake the pan once to settle everything together.
Cook mixture over medium-low heat without stirring for 3 minutes. We usually poke things around a bit during this step. The basic idea here is that you don't want to stir it and have all that grated cheese settle to the bottom.
Transfer oven and bake about 30 to 35 minutes, or until a knife inserted in the middle comes out clean. Allow to set for 10-15 minutes before cutting.
Notes
SKILLET: For best results, use a 10- to 12-inch cast iron or other heavy, oven-safe skillet.COOK TIMES: Cooking time for frittatas can vary dramatically, because it is so dependent upon the ingredients used and the density of the frittata. I’ve made frittatas that were done in 20 minutes, and others that took almost an hour to cook. To test for doneness, insert a knife in the center of frittata – it should come out clean. Allow the freshly baked frittata to set for 10-15 minutes before cutting.