Preheat oven to 350 degrees F (175 degrees C). Butter a 9x5-inch loaf pan.
Mix beef, onion, 1 cup ketchup, bread crumbs, eggs, salt, and pepper together in a bowl. Press beef mixture into a loaf in the prepared loaf pan.
Mix 1 1/2 cups ketchup, water, vinegar, brown sugar, mustard, and Worcestershire sauce together in a saucepan; cook and stir over medium heat until sauce is smooth and warm, about 10 minutes. Pour 1/3 the sauce over meatloaf.
Bake in the preheated oven, basting every 25 minutes with more sauce, until meatloaf is cooked through, 1 to 1 1/2 hours. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C). Cool in the pan for 10 minutes before slicing.
If you need to use two meatloaf pans to make the meatloaf fit comfortably in the pan with room to spare, do so.
My Aunt Libby's original recipe calls for 2 pounds ground beef but I usually use 1.3 to 1.5 pounds meat for my small family of three and adjust other ingredients accordingly. So one meatloaf pan is all I need.
Try not to substitute ground beef with turkey and use a fattier 80%-lean beef choice. True, it is not as healthy but the taste is amazing!
Per Serving: 534 calories;24.8 g fat;
48.4 g carbohydrates;
30.8 g protein;
153 mg cholesterol;
2215 mg sodium.