1. Preheat the oven to 350°F (175°C) to ensure an even and consistent bake.

Prepare the egg mixture:

1. In a large mixing bowl, whisk together the eggs, heavy cream, kosher salt, and black pepper until thoroughly combined. Fold in the Gruyère, fontina, feta, and Parmesan cheeses. Set this mixture aside.

Sauté the vegetables and aromatics:

1. In a 10-inch cast iron skillet, heat the olive oil over medium heat. Add the halved cherry tomatoes, sliced basil, red pepper flakes, and Italian seasoning. Cook for 4 to 5 minutes, or until the tomatoes begin to soften and release their juices.
2. Add the scallions and shallots to the skillet and sauté for an additional 2 minutes. Stir in the garlic and cook for 1 minute more, until aromatic.
3. Add the fresh baby spinach to the skillet and sauté until just wilted, approximately 1 to 2 minutes. Adjust seasoning with additional salt and pepper as needed. Reduce the heat to medium-low.

Combine and cook:

1. Pour the egg and cheese mixture into the skillet over the sautéed vegetables. Gently stir to distribute the ingredients evenly. Cook for 3 to 5 minutes, or until the edges begin to set.

Bake the frittata:

1. Transfer the skillet to the preheated oven and bake for 15 to 20 minutes, or until the center is fully set and no longer jiggly. Once baked, allow the frittata to rest in the skillet for 5 minutes before serving.

Serve:

1. Carefully loosen the edges and bottom with a rubber spatula. To unmold, place a large plate over the skillet, invert the frittata onto the plate, then flip it again onto a serving dish so the top is facing up. Slice and serve warm.