Preheat oven to 375 degrees F. Coat an 8-9″ round baking dish with cooking spray and set aside.
In a medium pan, heat olive oil over medium. When oil is hot, saute onions, until they turn translucent and are softened. Remove from the heat and spread on the bottom of the prepared baking dish. Top with tomatoes, mint, and cheese.
In a small bowl, whisk together eggs, half and half, and salt and pepper to taste. Pour over vegetables in baking dish.
Bake frittata for 25-30 minutes. Let cool for 2-3 minutes before cutting into it. Serve with a Greek salad.