Preheat the broiler. In a bowl, beat the eggs, then stir in the tomatoes, chopped pastrami, 1/4 cup parmesan and 3 tablespoons chives.
In a large nonstick skillet, melt the butter over medium heat. Add the onion and cook, stirring occasionally, until softened, about 5 minutes. Season with salt and pepper.
Increase the heat to medium-high, add the egg mixture and cook, stirring occasionally, until the frittata is set, 5 to 7 minutes. Sprinkle the remaining 1/4 cup parmesan on top. Transfer to the broiler and cook for 3 minutes more. Loosen the edges with a rubber spatula and slide the frittata onto a cutting board. Cut into quarters and top with the remaining 1 tablespoon chives.