Add ground beef, almond flour, salt, black pepper, allspice, nutmeg, garlic powder, onion powder, egg, and 2 tbsp heavy cream to a large bowl. Combine well using a spoon or your hands. Using your hands, roll up 15 meatballs.
Heat up a skillet over medium-high heat. Add 1 tbsp of butter and onion. Saute the onion until it starts caramelizing. It will take about 6-8 minutes. Remove from the pan and set aside.
Add olive oil to the skillet and cook the meatballs until they brown up and cook through. You'll want to turn them around while cooking so all of the sides cook evenly. It will take around 7-8 minutes.
Remove the meatballs from the pan onto the plate and cover with foil so they don't completely cool down.
Add the remaining 3 tbsp of butter into the skillet and return cooked white onion back into the skillet. Let it cook until it starts browning, then add the rest of the heavy cream, beef broth, and Dijon mustard. Stir well and let it simmer for 2-3 minutes.
In a small bowl, combine cornstarch with cold water. Add it into the skillet and mix it in. Continue to simmer while the sauce thickens up.
Add the meatballs back into the skillet, cover with sauce, and let them simmer for 2-3 more minutes before serving. Garnish with chopped parsley.