Step 1: Preheat the oven to 400°F.

Step 2: In a mixing bowl, whisk together the eggs, heavy cream, garlic powder, salt, black pepper, mozzarella, and Parmesan until well combined.

Step 3: Melt the butter in a cast iron skillet over medium heat. Add the diced red onion and sauté until it begins to soften.

Step 4: Stir in the chopped spinach and fresh basil, cooking just until the spinach wilts.

Step 5: Pour the egg mixture evenly over the vegetables in the skillet.

Step 6: Transfer the skillet to the oven and bake for about 5 minutes, allowing the eggs to begin setting.

Step 7: Carefully arrange the sliced tomatoes over the partially set eggs.

Step 8: Return the skillet to the oven and continue baking for another 10–15 minutes, or until the center is fully set.

Step 9: Let cool for a few minutes before slicing and serving.