In a medium bowl, whisk the eggs, Parmesan, parsley, ½ teaspoon salt, ⅛ teaspoon pepper, and the cayenne.
Heat 2 tablespoons of the oil in a 10-inch ovenproof nonstick skillet over medium-high heat. Add the potato and ¼ teaspoon salt and cook, stirring occasionally, until browned, 6 to 7 minutes. Transfer the potatoes to a bowl. Reduce the heat to medium. If the pan is dry, add the remaining 1 tablespoon oil. Add the onion and cook, stirring frequently, until it softens and begins to brown, 4 to 5 minutes. Stir in the asparagus, garlic, ¼ teaspoon salt and ⅙ teaspoon pepper. Cook, stirring frequently, until the asparagus is bright green and crisp-tender, 3 to 4 minutes. Lower the heat to medium-low and add the egg mixture and potatoes, stirring until the ingredients are combined, 10 to 15 seconds. Add the Cheddar and stir until mixed in.
Cook without stirring until eggs have almost set, 10 to 12 minutes. (The center may still be loose but should be bubbling a little; the sides should be set). Meanwhile, position an oven rack 6-inches from the broiler element and heat the broiler to high.
Transfer the skillet to the oven and broil until the eggs have set completely and the top of the frittata is golden brown, 1 to 3 minutes. Let rest for 5 minutes. Gently transfer to a cutting board, slice into 4 wedges and serve.
Notes
If you do not have an ovenproof skillet, wrap the handle of the pan with foil before placing in the oven.
If you're counting WW Points be sure to use reduced fat cheddar cheese in your frittata.