Place potatoes in a medium saucepan, and cover with water. Bring to a boil. Reduce heat, and simmer until tender when pierced with a sharp knife, 10 to 12 minutes. Drain; let cool. Peel, and cut into 1/4-inch-thick slices.

Preheat oven to 400 degrees. Heat oil in an ovenproof 10-inch nonstick skillet over medium-high heat. Cook onions until just translucent, about 2 minutes. Add potatoes, and cook, stirring occasionally, until onions and potatoes are golden brown, 4 to 5 minutes. Season with salt and pepper.

Mix together cheddar and eggs. Season with salt and pepper. Reduce heat to medium. Pour egg mixture into skillet. Dollop with sour cream, and swirl into egg mixture using the tip of a knife. Cook until edge is set, about 2 minutes. Carefully transfer skillet to oven, and bake until eggs are completely set, 10 to 12 minutes. Invert frittata onto a plate. Let cool slightly. Serve warm.