1) Preheat broiler and put a large pan of water on to boil. Heat an 8 to 10-inch ovenproof skillet coated with an olive oil mister or non-stick spray over medium-high heat.
2) Sauté the onions until browned, about 8 to 10 minutes; set aside. While onions are cooking, place bunches of Swiss chard by the stems into the boiling water, leaving the stem bunch up out of the water. Prepare a bowl with ice water. Blanch chard for two or three minutes, then transfer the chard by the stems to the ice water and drain. Squeeze out excess water, and chop finely until the leaves stop.
3) In the skillet, sauté potatoes until golden brown, about 8 to 10 minutes. While the potatoes are browning, whisk the eggs, salt, and pepper together in a medium bowl until frothy. Mix in sautéed onions, browned potatoes, and the finely chopped Swiss chard.
4) Heat the oiled skillet over medium-high heat and pour the egg mixture into pan. Cover the pan and cook 3 to 5 minutes or until set.
5) Spread cheese on the frittata. Transfer the skillet to the oven and broil about 3 minutes until browned.