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.