Potato, Tomato, and Smoked Mozzarella Frittata

1. Preheat the oven to 400°F. Melt the butter in a 9-inch ovenproof skillet over medium-high heat. Add the potatoes and cook for 8 to 10 minutes, until the potatoes are golden brown and tender. Using a slotted spoon, transfer the potatoes to a small bowl and set aside. Add the red onion and tomatoes to the pan, season with salt and pepper, and cook for 2 to 3 minutes, until the vegetables are softened. Stir in the spinach and garlic and cook for about 1 minute, until the spinach is wilted. Add the potatoes back to the pan, stirring to combine.

2. Meanwhile, whisk together the egg whites, eggs, heavy cream, oregano, and 1 cup of cheese. Season with salt and pepper and add to the skillet. Reduce the heat to medium and cook for 5 to 6 minutes, until the eggs begin to set and the edges begin to brown. Sprinkle the remaining 1/2 cup of cheese over the top and transfer to the oven. Cook for 15 to 20 minutes, until the top is golden brown and the eggs are set. A knife inserted into the frittata will come out clean. Remove from the oven and let cool for 10 minutes. Run a rubber spatula around the edges of the pan to loosen the frittata. Slide the frittata onto a platter and serve warm or at room temperature.