Preheat oven to 375 degrees and spray a 9" pie dish with cooking spray.
Place a skillet over medium heat and melt butter. Add onions and cook approximately 3–4 minutes until soft and translucent.
In a large mixing bowl, whisk flour together with one egg. Add remaining eggs and milk and beat until light and fluffy. Add cooked onions, basil, cherry tomatoes, 1 cup of cheese, salt and pepper and stir until well combined.
Transfer the filling to prepared pie dish and top with remaining cheese. Bake 30–35 minutes or until a knife inserted in the middle comes out clean. Serve immediately with a light side salad.
One Last Thing
Don’t be alarmed if your frittata grows larger than your pie pan. As it cools it will settle back into place.