Place the eggs in a medium bowl and add the cheese, along with ½ teaspoon salt and several grinds of pepper, and stir to combine and disperse the cheese. Pour the egg mixture over the potatoes and onions and reduce the heat to medium, swirling the pan so that the eggs even out. Cook until the edges are set, a couple of minutes depending on the heat level, checking to make sure that the eggs aren’t sticking. Place the pan underneath the broiler until the eggs are just set, a minute or two depending on your broiler. When the top is set, invert the frittata onto a cutting board, cover with the diced avocado, if using, and cut into wedges. Serve.