1. Preheat the oven to 350 degrees F
2. Place the butter or oil in a medium-to-large ovenproof skillet, preferably non-stick, and turn the heat to medium. While it's heating, beat together the eggs, cheese, salt, and pepper. When the butter melts or the oil is hot, pour the eggs into the skillet and turn the heat to medium-low. Cook, undisturbed, for about 10 minutes, or until the bottom of the frittata is firm.
3. Transfer the skillet to the oven. Bake, checking every 5 minutes or so, just until the top of the frittata is no longer runny, 10 to 20 minutes more.(To speed things up, turn on the broiler, but be careful not to overcook.) Garnish and serve hot or at room temperature.