In a microwave-safe dish, place a slice of cheese. Later, place the chorizo, onion, oregano, pepper to taste and cover with another slice of cheese. Cook in the microwave until the cheese melts. (Microwave power levels vary, so be careful not to burn the cheese.)
Whip the egg and season to taste. Make an omelet with the egg. Set it aside.
Toast the bread and spread mayonnaise on each slice of bread. Place the chorizo and melted cheese on each slice of bread.
Place the omelet, lettuce and tomato and cover with the remaining slice of bread.
Cut into two portions in the shape of a triangle and serve immediately.