Preheat oven to 350 degrees.
In a skillet, heat butter over medium-high heat. Add the chopped onion and cook for 5-7 minutes or until the onion is tender, frequently stirring to avoid burning. Add the minced garlic to the onion, and continue sautéeing for 30 seconds or until fragrant.
In a large bowl, combine the ground beef, egg, sautéed onion, milk, Worcestershire sauce, bread crumbs, Ritz crackers, parsley, salt, and pepper with your hands. Place the mixture in a lightly greased 9-by-5-inch loaf pan, patting down into an even layer.
In a small bowl, combine the ketchup, light brown sugar, and red wine vinegar. Mix well and spread evenly over the top of the meatloaf.
Cover the meatloaf with aluminum foil and place it in the preheated oven for 45 minutes. After 45 minutes, uncover and cook for an additional 20 minutes. Remove the meatloaf from the oven and let it sit for 15 minutes before slicing. Enjoy!
If you don't have a loaf pan on hand or would prefer not to use one, form the meat mixture into a loaf and place it into a lightly greased 9-by-13-inch baking dish.