Soak oats in milk for 5 minutes. Stir in egg, onion, Worcestershire, thyme, salt and pepper, then fold in beef and pork.
Shape into a 7-by-4-inch loaf about 2 1/2 inches high on a sturdy foil sling with slightly raised edges. Add 1 cup water to a 6-quart electric pressure cooker and set a trivet inside.
Lower the loaf on its sling onto the trivet, keeping it above the water. Lock the lid and pressure-cook on high for 30 minutes, allowing about 15 minutes for the cooker to reach pressure.
Allow a 10-minute natural release, then release remaining pressure as the manufacturer directs. Check the center for 160°F; if lower, pressure-cook for 3 minutes more with a 5-minute natural release and recheck.
Lift the loaf on its sling onto a rimmed metal pan. Mix tomato paste, balsamic vinegar, honey and 1 tablespoon water and spread over the top.
Broil 6 inches from the heat for 2 to 4 minutes until the coating is tacky, watching constantly. Rest for 10 minutes and cut into 6 slices.