1. Preheat the oven to 400 degrees Fahrenheit and adjust an oven rack to the middle position.
2. Place butter or olive oil in a 10.25 Inch Skillet over medium-low heat for 4-5 minutes. Tilt pan to coat sides with oil. While pan is heating, set aside half basil leaves and finely chop remaining half. In a large mixing bowl, whisk together eggs, milk or cream, salt, pepper, and chopped basil until well blended.
3. Pour half of the egg mixture into the skillet and cook 3-4 minutes, until bottom has just begun to set. Carefully place half of the sliced tomatoes into the eggs. Pour in the rest of the egg mixture, and place remaining tomatoes on top (they may sink slightly). Cook another 4-5 minutes on the stovetop, then place in oven.
4. Bake until the top is set and sides begin to pull away from the skillet, about 8-10 minutes. Remove skillet from oven, carefully place mozzarella slices on top, and let cool for 5 minutes as cheese melts. Use a spatula to remove frittata from skillet. Garnish with remaining basil and serve immediately.

Note: This recipe can also be made in a 12 Inch Skillet – simply shorten the baking time to 4-7 minutes