Heat the oven to 400°F and line a rimmed sheet pan with parchment. Soak rye crumbs in milk for 5 minutes.
Stir egg, onion, caraway, salt and pepper into the soaked crumbs. Fold in beef and pork until evenly combined.
Divide into 6 equal portions and shape into 4-by-2-inch ovals about 1 1/2 inches thick, leaving 2 inches between them.
Bake for 15 minutes. Mix mustard, honey and vinegar and brush thinly over each loaf.
Bake for 10 to 15 minutes more until each center reaches 160°F.
Rest for 5 minutes and serve one loaf per portion.