Spanish-Style Potato and Onion Frittata

Ingredients

• large eggs
• Kosher salt and freshly ground pepper
• grated Parmesan, divided
• pound yellow potatoes, peeled and cut into small dice
• tablespoons extra virgin olive oil, divided, plus more for garnish
• tablespoons unsalted butter
• medium yellow onion, finely chopped
• shredded mozzarella or other melting cheese
• Sour cream, for serving