Place 1 tsp of the olive oil in a skillet over medium heat. Saute the onions in the oil until they are soft.

Add ½ of the onions to a bowl with the flour, worcestershire sauce, 1 cup of the beef broth, the ground mustard, 1 tbsp of the tomato paste, and the red wine vinegar. Whisk to combine and set aside.

In a large bowl, mix together the beef, breadcrumbs, egg, 1 tbsp tomato paste, ¼ cup of broth, the remaining onions, and salt and pepper to taste. Knead with your hands to combine, and form into 20 meatballs.

Heat the remaining oil in a skillet over medium-high heat, and sear the meatballs on both sides evenly.

Reduce the heat to medium-low and mix in the mushrooms and sauce mixture.

Cover and simmer for 6-8 minutes, or until the sauce is thickened.

Enjoy!