Preheat your oven to 375.

Add the eggs, milk, salt and pepper to a medium bowl, and beat until combined.

Stir in half of the cheese into the egg mixture, and set aside.

Spray a 10 inch non-stick skillet with cooking pray, and preheat over medium low heat.

Add the diced onions, and cook until translucent, about 3 minutes.

Place the tomato slices in the pan.

Pour egg mixture over the tomatoes and onion.

Cook on the stovetop on medium/ low heat until the edges start to set, about 4 minutes.

Use the spatula to loosen and pull the edges inward, allowing more liquid eggs to move to the hot pan.

Repeat this several times. (I went around the pan about 3 times)

Remove the pan from the heat.

Place the basil leaves on the top, and sprinkle with the other half of the cheese.

Place the skillet in the oven until the center sets. Mine took 15 minutes.

Remove the skillet from the oven, and let it sit about 5 minutes to cool slightly.

Use the spatula to transfer to a serving dish, if desired. (This is when you will high five yourself for using non-stick!)

Use a knife to cut the frittata into eight servings. I make a giant plus sign (cuts into fourths), and then cut each of those pieces in half.

Serve immediately. Save leftovers in the refrigerator for up to a week.