• 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.