How to Make It
Preheat oven to 400°F. Whisk eggs, dairy and ⅔ of cheese in a medium bowl until evenly combined; season with salt and pepper. Set aside.
Heat 1 tablespoon olive oil (or melt 2 tablespoons unsalted butter) in a 10– to 12-inch cast-iron skillet over medium-high heat. Add meat, if using, and cook until browned and crispy, 5 to 7 minutes. Transfer to a plate using a slotted spoon. To any skillet drippings, add onions and cook, stirring until softened, 5 to 7 minutes; season with salt and pepper. Add vegetables and stir to combine.
Add egg mixture to skillet and quickly stir to combine. Sprinkle remaining cheese over top and transfer skillet to the oven. Bake until frittata is puffed and almost set (it should still wiggle a little).
Increase heat to broil. Broil frittata until golden in spots and just set, 1 to 2 minutes (but keep an eye on it as broilers vary greatly). Let cool 5 minutes before slicing or scooping and serving.