Finely chop 1 oz of the mushrooms into small pieces and set aside. Heat a large pot on medium, add 1 tsp oil and onions and cook until golden brown, about 4 to 5 minutes. Divide onions in two.
In a small bowl, blend flour and 2 cups broth until smooth. Mix in remaining onions, remaining 2 tbsp tomato paste, vinegar, Worcestershire sauce and mustard powder.
Gently shape into 20 small meatballs. Place on baking sheet lined with parchment paper.
Bake at 400 for 20-25 minutes until done.
Meanwhile, add the mushrooms and sauce to the pan, cover and simmer while the meatballs bake.
When the meatballs are done, add them to the pot and stir to coat.
Garnish with parsley.
Notes
Can serve over brown rice, mashed potatoes, or cauliflower mash.