Turn on your oven to broil. In a large skillet, melt the butter and add the onion and saute for a couple minutes until the onion is tender and translucent. Add the mushrooms and chorizo sausage and cook for a couple more minutes. Add tomatoes and stir. Season with salt and pepper.
Beat the eggs in a bowl. Pour the eggs over the mushrooms and sausage.
Cook the eggs, for just a minute and sprinkle the cheddar cheese over the top.
To finish this frittata, place the skillet in the oven under the broiler for a couple minutes, just until the top is nice and golden. Garnish with chives or green onions.