How to Make It
Preheat oven to 400°. In a medium cast-iron or nonstick frying pan over medium heat, sauté onions in oil until soft, about 5 minutes. Meanwhile, combine eggs, salt, and pepper in a bowl and beat lightly just until blended.
Pour egg mixture into frying pan and sprinkle in gruyère, sage, and rosemary. Stir gently with a wooden spoon until mixture begins to set, about 2 minutes.
Cover frying pan, transfer to oven, and bake until frittata is puffed and set, 12 to 14 minutes. Cut into 6 slices, and serve immediately, sprinkled with basil.