Preheat oven to 350 degrees. Preheat cast iron skillet on over medium heat (no oil).
In a small bowl, lightly beat eggs just until yolk and white are combined.
Add milk, vegetables, herbs, and half the cheese. (Good melting cheeses include Fontina, gruyere, cheddar. Goat and Feta cheeses also work well.)
Carefully brush the hot skillet with olive oil. Pour in the egg mixture. In 2 to 3 minutes, when the egg mixture pulls away from the sides of the skillet, top with remaining cheese and move skillet to the preheated oven. Bake about 5 to 8 minutes. Remove from oven when the center is still a little jiggly. The skillet is so very very hot, the frittata will continue cooking when removed from the oven. If you bake the frittata until the egg is completely set, your frittata will be overcooked.
Notes
Vegetables: Greens like spinach and chard, tomatoes can go in raw. Mushrooms, potatoes, peppers and other vegetables that take longer than a couple minutes to cook need a quick saute to soften.