Ingredients: For the Filling: 2 tablespoons extra virgin olive oil 2 tablespoons unsalted butter 1 large sweet onion, cut into ½-inch dice (about 1½-2 cups) 2-3 large carrots, peeled and cut into ½-inch dice (about 1½-2 cups) 2 stalks celery, chopped (about 1 cup) 1 teaspoon Kosher …