To make the frittata: Preheat the oven to 350 degrees F. Heat the oil in a 10-inch nonstick skillet over medium heat. Add the onion and garlic and cook, stirring, for 3 minutes or until tender. Stir in the spinach. Reduce the heat to low. In a large bowl, beat the eggs and egg whites with the milk until light yellow and frothy. Pour the egg mixture over the spinach in the skillet. Cook for 5 to 7 minutes, until the egg mixture is cooked on the bottom and almost set on top. Sprinkle with the cheese. Bake in the oven until the eggs are set and the cheese has melted, 5 to 10 minutes.
To make the salsa: In a large bowl, stir together the tomatoes, scallions, garlic, cilantro, limejuice, salt, and pepper. Serve fresh, at room temperature, over the frittata.