a large skillet, heat the oil and the butter over medium-high heat. add the meatballs and cook, turning until brown on all sides. transfer to a 2 1/2 quart baking dish.
make the sauce by combining all the ingredients in a medium bowl, whisking to dissolve the sugar and combine. adjust the seasoning, to taste. pour the sauce over the meatballs and bake until the meat is cooked through and the sauce is bubbly, 20 minutes. remove from the oven and serve immediately. 