Break eggs into a large bowl and beat with a whisk. Add salt, pepper and any fresh herbs.
Heat a 10-inch skillet with an oven proof handle over a medium heat on the stove top. Add olive oil and onions, stirring until onions are soft and starting to brown. Add garlic and cook for 1 minute.
Add vegetables and saute for 2 minutes.
Add vegetables to the egg mixture and stir in cheese (if using).
Clean and dry the skillet and add return to a medium heat. Add 1 tablespoon of olive oil once hot and swirl to coat the skillet.
Pour in the egg/vegetable/cheese mixture and cook undisturbed over medium heat for 3-4 minutes until eggs are set on the bottom.
Transfer skillet to pre-heated broiler and broil for about 3-4 minutes, until frittata is firm in the center.
Serve warm or at room temperature.