To make olhas varenyky perogies, you'll need: 14 cup (60 ml) sour cream, 2 tablespoons melted butter, 1 teaspoon kosher salt, and 1 tablespoon vegetable oil. Heat the oil in a large skillet over medium-high heat. Add the onion and cook, stirring frequently, until softened, about 5 minutes. Stir in the flour and salt. Cook, whisking constantly, for 1 minute. Remove the pan from the heat and add the milk. Bring to boil, then reduce heat to low and simmer for 30 minutes, or until thickened. Serve with ketchup.