Potato Parsnip Rosti with Caramelized Onion Sour Cream

Name: Potato-Parsnip Rosti with Caramelized Onion Sour Cream
Ingredients: 1 medium yellow onion; 2 tablespoons finely chopped fresh chives; 2 tablespoons unsalted butter; 3/4 teaspoon kosher salt, divided; 1/2 teaspoon freshly ground black pepper, divided; 3/4 cup sour cream; 1/2 medium lemon; 1 pound russet potatoes (about 3 medium); 1 medium parsnip (8 to 10 ounces); 1/2 medium yellow onion; 5 tablespoons unsalted butter, divided; 2 teaspoons kosher salt; 1/2 teaspoon freshly ground black pepper; 2 tablespoons olive oil, divided