How to Make It
Heat oven to 400° F. Toss the potatoes, 3 tablespoons of the oil, and ¼ teaspoon each salt and pepper on 2 rimmed baking sheets. Roast, tossing once, until golden brown and cooked through, 15 to 18 minutes.
Meanwhile, heat the remaining tablespoon of oil in a medium ovenproof nonstick skillet over medium-high heat. Add the onion, garlic, and ¼ teaspoon each salt and pepper and cook, stirring occasionally, until golden brown, 8 to 10 minutes.
Add as much kale to the skillet as will fit. Cook, tossing and adding more kale when there is room, until tender, 5 to 7 minutes. Remove from heat. Stir in the eggs and ¾ cup of the Manchego; top with the remaining ¼ cup of Manchego. Transfer the skillet to oven. Bake until almost set, 6 to 8 minutes. Switch oven to broil and broil until golden brown and set, 1 to 2 minutes.
Serve the frittata with the potatoes. Drizzle the potatoes with the hot sauce (if desired).