Finely chop 1 oz of the mushrooms into small pieces and set aside. Heat the Instant Pot on saute (or a large nonstick pot on medium-high) and add 1 tsp oil and onions and cook until golden brown, about 4 to 5 minutes. Divide onions in two.
In a large bowl, combine half of the sautéed onions with the ground beef, ground turkey, chopped mushroom, bread crumbs, egg, 1 tbsp tomato paste, 1/4 cup of the beef broth, 3/4 tsp salt and black pepper.
In a small bowl, blend flour and 1 cup broth until smooth. Mix in remaining onions, remaining 1 tbsp tomato paste, vinegar, Worcestershire sauce and mustard powder.
Gently shape into 20 small meatballs.
Stove Top Method:
Heat a large deep skillet over medium heat, add the remaining teaspoon of oil and brown the meatballs, in batches as needed, about 2 minutes until no longer sticks (if it sticks, it's not ready to turn yet). Flip and brown an additional 2 minutes. Add the mushrooms, 1/8 teaspoon salt and black pepper and pour the sauce over the meatballs, cover and cook low heat for 25 minutes. Garnish with parsley.
Slow Cooker Method:
Saute meatballs in a skillet with the remaining teaspoon of oil, then transfer to slow cooker with mushrooms, salt and pepper and pour sauce over. Cover and cook low 6 to 8 hours, until tender.
Instant Pot Method (see note below)
Heat the Instant Pot on saute, add remaining teaspoon of oil and brown the meatballs (in batches so you don't overcrowd the pot) about 2 minutes until no longer sticks (if it doesn't turn, it's not ready yet), turn and brown an additional 2 minutes. Add the mushrooms, 1/8 teaspoon salt and black pepper and pour the sauce over the meatballs, cover and cook on high pressure 10 minutes. Quick or natural release. Garnish with parsley.
*some people have issues with the meatballs sticking to the instant pot when browing. If this is an issue, you can brown in a skillet.