Set oven to the broil setting. Line a baking sheet with parchment paper or aluminum foil.
Combine ground beef, buttermilk, egg, breadcrumbs, garlic, parsley, onion powder, allspice, nutmeg, black pepper, and salt in a large bowl. Mix until all of the ingredients are combined.
Use an ice cream scoop (or Spoon out 3 tbsps) to scoop out ground beef mixture and roll into a meatball. Repeat until ground beef is used up. Place meatballs onto the prepared baking sheet and broil on the middle oven rack for for 10 minutes, until browned.
Whisk together half and half, cornstarch, and beef bouillon paste together in the bottom of the slow cooker, then add the remainder of the gravy ingredients (except the sour cream) - dijon mustard, worcestershire sauce, beef broth, and thyme. Whisk together until combined.
Place meatballs in a slow cooker and add the onions. Cover with the lid and Cook on high for 3 hours, until the meatballs are cooked all the way through, stirring occasionally. Remove the lid and continue to cook for 60 minutes to allow the sauce to thicken. Add in the sour cream and stir until combined.
Serve with mashed potatoes or noodles.
Notes
For a thicker sauce, after the meatballs are cooked all the way through, remove the lid and continue to cook until the sauce is at your desired consistency.
Don’t overwork the meat mixture: Mix gently just until combined—overmixing leads to tough meatballs.
Stir in sour cream at the end - Adding sour cream too early can cause it to curdle. Stir it in during the last 15–30 minutes so the sauce stays smooth, rich, and luscious.
Brown the meatballs first. Even though the crockpot does the slow cooking, quickly searing the meatballs in a pan first adds that golden crust and deep umami flavor. Totally worth the extra step.