Chop fine 1oz mushrooms and and set aside.
Heat Instant Pot using the saute function, add 1 tsp oil and cook until golden brown about 4-5 minutes.
In a large bowl, combine the sautéed onions with the ground beef, chopped mushrooms, bread crumbs, egg, 1 tbsp tomato paste, 1/4 cup beef broth, 3/4 tsp salt and pepper.
Gently shape into 20 meatballs about the size of a golf ball.
In a small bowl, stir 1 cup beef broth into the flour until smooth. Mix in remaining tomato paste, vinegar, Worcestershire and mustard powder. Set aside.
Heat the Instant Pot on sauté and add remaining oil and brown the meatballs in batches to not overcrowd the pot about 2 minutes then turn and do the other side. If the meatballs don't turn easily its not ready to be turned.
Add the mushrooms, 1/8 tsp. salt and black pepper to the pot with the meatballs then pour the broth/flour sauce over the meatballs. Ste Instant Pot to manual, set the Instant Pot to sealing and cook on high pressure for 10 minutes. Let the pressure release on its own (NR or natural release. Garnisg with parsley.