Preheat oven to 375°F with a rack in the center.

In a medium mixing bowl, whisk the eggs together with the dairy, salt, and pepper. I like to use a fork instead of an actual whisk, since it combines the ingredients without adding excess air.

Heat the fat in a 12-inch skillet over medium-high.

Finely dice onion or shallot or cut leeks into thin half-moons. Add to pan and cook until softened.

Chop meat, if using, into bite-sized pieces. The meat should be cooked by the time it goes into the frittata, so if it's raw when you're starting, cook it in the pan now. If it leaves more than about a tablespoon of fat in the pan, drain the fat before proceeding.

Chop vegetables into bite-sized pieces. Just like the meat, most vegetables should be cooked by the time they go into a frittata. If they aren't yet cooked, give them a sauté in the frying pan at this point.

With the cooked meats and vegetables in the pan and the pan set over medium heat on the stove, pour in the beaten egg mixture and sprinkle with the cheese and capers or olives, if using. Let the frittata cook undisturbed until the bottom is set, about a minute.

Transfer the pan to the oven and bake until eggs are just set, about 10 minutes.

Let cool slightly, or completely if you prefer, before cuting into wedges.