liver and onions recipe

- 1 pound beef liver, sliced - 1 cup milk (for soaking) - 2 large onions, thinly sliced - 4 tablespoons butter, divided - 2 tablespoons olive oil - 1/2 cup all-purpose flour - 1 teaspoon salt - 1/2 teaspoon black pepper - 1 teaspoon paprika (optional) - Fresh parsley, chopped (for garnish, optional)