For the Meat Mixture:
1/2 small onion
1 stalk celery
1 medium carrot
2 pounds ground beef
2/3 cup panko breadcrumbs
1/2 cup tomato ketchup
1/2 cup milk
2 large eggs
1 1/2 teaspoons kosher salt
1/2 teaspoon freshly ground black pepper
1/2 teaspoon Worcestershire sauce
For the Topping:
2/3 cup tomato ketchup
1/3 cup packed brown sugar
1 1/2 tablespoons Dijon or yellow mustard
1/4 teaspoon Worcestershire sauce