In a large skillet, add the butter and oil and heat until melted and combined. Add the onions and cook over a medium high heat for 3 minutes. Add the red bell pepper and zucchini and continue to cook for up to 15 minutes or until browned. Stir often. When cooked, turn the heat to medium and add the garlic and cook for 30 seconds more. Add the salt, pepper and red pepper flakes if using. Stir to combine.