Meanwhile, in a small bowl, combine the beef, sausage, egg, mayonnaise, pork rinds, parmesan, and salt and pepper.
Form into 1 to 2 tbsp large meatballs.
Heat your oil and butter in a skillet over medium-high heat.
Add the onions to the skillet and cook until browned all over, turning every 2 minutes.
It will take about 8-10 minutes to cook the meatballs, so I use this time to measure out my other ingredients and chop the vegetables.
Once the meatballs are cooked, remove from the pan and set aside.
Add the chopped onions to the frying pan to saute.
Cook the onions until translucent, then add the garlic and mushrooms. Cook for 2 minutes, then add the beef broth.
Stir in the sour cream and dijon mustard, and Worcestershire sauce to taste.
Add the meatballs back in and allow the sauce to cook and thicken, 3-5 minutes. If you want the sauce thicker, remove 1/2 cup of the sauce and whisk a tbsp of arrowroot powder to that sauce, then stir back into the skillet.