Preheat the oven to 400F. Heat the oil in an oven safe skillet over medium-high heat. Add the onion and chorizo. Cook, stirring, until the onion has caramelized and chorizo has softened and rendered. Transfer the mixture to a bowl and set aside.
Place the tortilla in the same pan and cook until it begins to puff up, then flip over. Top with onion mixture, 1/4 cup cotija cheese and tomatoes.
Beat eggs with salt and pepper and pour over tortilla. Top with remaining cheese. Cook until the edges of the eggs have set.
Transfer pan to oven and cook for an additional 10 to 12 minutes, or until the eggs have cooked through and the cheese is golden.