Set oven to the broil setting. Line a baking sheet with parchment paper or aluminum foil.
Whisk together all of the gravy ingredients except the beef bouillon cube - half and half, cornstarch, dijon mustard, worcestershire sauce, beef broth, and thyme. Set aside.
In a small bowl, mix together whisk together an egg and buttermilk. In a large mixing bowl, add ground beef, breadcrumbs, garlic, parsley, onion powder, allspice, nutmeg, black pepper, and salt. Mix together and then add egg/buttermilk mixture. Continue to mix together until all ingredients are incorporated.
Spoon out 3 tbsps of the ground beef mixture and roll into a meatball. Repeat until ground beef is used up. Place meatballs onto the prepared baking sheet and broil for 6 minutes.
Place meatballs in a slow cooker. Pour gravy mixture on top and add the beef bouillon cube, onions, and mushrooms. Cook on low for 6-8 hours, stirring occasionally.
Serve with mashed potatoes or noodles.