In a large skillet, heat the oil until shimmering. Add 1/2 cup of the onion, the chiles and a generous pinch of salt and cook over moderate heat, stirring, until the onion is just starting to brown, 8 minutes. Add the Swiss chard and cook, stirring, until softened, 5 minutes. Stir in the eggs and tomatoes and cook over moderately low heat, stirring, until the eggs are just cooked, 5 minutes. Stir in the remaining 2 tablespoons of onion and the cilantro and season with salt and pepper. Serve the eggs in warm tortillas.