In a large bowl, combine onion, bread crumbs, ½ cup ketchup, eggs, worcestershire, 1¼ tsp salt, ¾ tsp pepper, beef, veal and pork. Mix with your hands until just combined.
Transfer meat mixture to a 2 lb loaf pan and set aside.
Peel potatoes; dice into 1 1/2" pieces. Add potatoes to bottom of pressure cooker inner pot and pour in 1 cup of water.
Place loaf pan on top of potatoes. Lock lid in place and make sure vent is set to SEAL.
Cook at high pressure for 30 minutes. Natural release for 5 minutes, then quick release remaining pressure. Remove loaf pan.
In a small bowl, mix remaining 1/4 cup ketchup with 2 tablespoons brown sugar. Spread over top of meatloaf. Broil in oven for 5-7 minutes while you finish the potatoes.
Use a potato masher to mash potatoes and butter together in inner pot. NOTE: I did not have any water left in my pot, if you do, feel free to drain the potatoes and return to the pot. Use a hand mixer to whip potatoes, adding up to 1 cup of cream of milk to achieve your preferred texture. Season to taste with salt and pepper.