Preheat oven to broil.

Brown chorizo over medium-high heat, using a wooden spoon to break up the chorizo into small pieces. Add onions and cook until translucent. Drain off excess fat and spread into the bottom of an oven-proof medium-sized casserole dish, reserving 1 tablespoon.

Place cheese and canned tomatoes in a microwave-safe bowl and microwave on high power, stopping and stirring every 15 seconds, until the cheese has melted and combined with the tomatoes. Spread cheese into the casserole over the chorizo.

Adjust top oven rack so that it is 6 inches below the broiler element. Place dish on top rack and broil until cheese stars to turn golden brown, 5-10 minutes.

Remove from oven and sprinkle with reserved meat. Serve hot.