Preheat your oven to 325°F (lower temp gives a custardy texture).
Beat eggs, cream, salt, and pepper in a bowl until combined.
Stir in half the mozzarella and set aside.
Spray a 10-inch oven-safe nonstick skillet with cooking spray and heat over medium-low.
Cook diced onions until translucent (about 3 minutes).
Arrange tomato slices in the pan.
Pour egg mixture over tomatoes and onions.
Cook on stovetop until edges start setting (about 4 minutes).
Top with basil leaves and remaining cheese.
Bake for 20-23 minutes until center is set.