In a medium pan, cook onion over medium heat in 1/2 Tbsp olive oil until soft, 5 minutes. Remove from pan.
Combine cooked onions, ground beef, 8oz mushrooms, egg, allspice, nutmeg, and panko in a large bowl. Mix together and form balls, you should be able to make about 24 with this mixture.
In the same pan, on medium - high heat, add 1 Tbsp oil and half the meatballs. Cook 5 minutes on each side, remove from pan. Repeat with the remaining meatballs. Remove from pan.
Add mushrooms to pan and cook until soft, another 5 minutes. Remove from pan.
Melt butter, and add flour, whisk to combine. Slowly add in beef broth, about 1 cup at a time whisking the sauce until thick in between every cup. Add heavy cream, and parsley. Simmer for 5 minutes.
Add meatballs back into pan, cook 10 minutes uncovered.
Serve over egg noodles.