Heat vegetable oil in a nonstick skillet, over medium heat. Saute onion and serrano pepper until softened, about 1-2 minutes. Season with salt and black pepper.
Add tomato, and cook for 1 minute.
Add eggs, cracked one at a time, on top of the onion tomato mixture. Continue to cook, stirring constantly, until eggs are cooked to taste, about 3 to 4 minutes.
Serve immediately with warm tortillas or crumbled tortilla chips over top.