Whisk together the eggs and milk in a large bowl. Season with onion salt, garlic powder and black pepper.
Heat an oven proof large pan like a cast iron pan over medium heat. Melt butter and swirl around the sides. Pour the eggs mixture and cook for about 2 minutes then top with tomatoes, basil and cheese. Cook for about 2 more minutes until the bottom starts to brown and egg is slightly firm.
Transfer to the oven and cook until set about 1 to 2 minutes. Watch carefully so it doesn't burn. Remove from the oven and let stand for 10 minutes before slicing. Serve
Recipe by The Gluten Free at https://www.theglutenfree.com/gluten-free-frittata-2/