In a large bowl, combine ground beef, egg yolks, tomato paste, mustard, Worcester sauce, gluten free breadcrumbs; season with salt, pepper and garlic powder. Using your clean hands, mix well and shape meatballs. You should get about 20 meatballs.
Melt 2 Tbsp butter in a large skillet over medium heat. Add meatballs to skillet in two batches and cook on all 4 sides for about 2 minutes per side. Add more butter if needed. Remove meatballs from the skillet and set aside on a paper towel-lined plate.
Add the remaining 2 Tbsp butter in the skillet. Add onion and mushrooms to the skillet, and cook until vegetables are soft, for about 3 minutes.
In a small bowl, mix together 1/2 cup beef broth and arrowroot starch. In the skillet, whisk in remaining 1 cup beef broth, Worcester sauce, and tomato paste until well combined, about 1-2 minutes. Bring to a boil, reduce heat to simmer and stir in arrowroot starch mixture. Cook until the sauce has thickened, about 1-2 minutes. Add more beef broth as needed until desired consistency is reached. Stir in meatballs.
Serve warm over egg noodles, mashed potatoes or mashed cauliflower, garnished with parsley, if desired.