Preheat the oven to 400°.
Coat a 8-inch cast iron skillet with a thin layer of olive oil. Heat tomato, basil and garlic on the stove in the skillet over medium-high heat. Once the tomatoes become soft and fragrant, reduce the heat to medium and pour the eggs into the skillet. Stir to combine.
Distribute the mozzarella balls throughout the dish. Cook on the stove until the edges set but the center is still uncooked, roughly 10 minutes. Move the dish to the oven and cook for 15 minutes until the center is set and the top has browned.
Allow the frittata to cool until it can be transferred to a plate. Slice and serve.