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 until soft and translucent, approximately 3-4 minutes.
In a large mixing bowl, whisk flour together with one egg, then add remaining,eggs and milk, then beat until light and fluffy. Add cooked onions with 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 and crusty bread.
Notes
Taking out the crust is a simple and easy way for cut the carbs!