Step 1

Preheat oven to 350°F.

Step 2

Finely chop onion. Cut enough bread into 1/2-inch cubes to measure 1 1/4 cups. In a large bowl lightly beat egg. Add chuck, onion, bread cubes, 1 cup tomato sauce, salt, and pepper and lightly blend with hands until just combined. In a roasting pan form mixture into a roughly 9- by 5-inch loaf and bake in middle of oven 10 minutes.

Step 3

While meatloaf is baking, in a small saucepan whisk together water, vinegar, brown sugar, mustard, and remaining cup tomato sauce and bring to a boil over moderate heat.

Step 4

After meatloaf has baked 10 minutes, spoon enough sauce over meatloaf to coat it. Continue to bake meatloaf, spooning more sauce over it after 30 minutes, 1 hour and 10 minutes more, or until a thermometer inserted in center registers 160°F. Let meatloaf stand in pan on a rack 10 minutes. Return sauce remaining in saucepan to a boil and simmer 1 minute.

Step 5

Serve meatloaf with sauce on the side.