Prepare Meatball Mixture by combining ground beef, ground pork, breadcrumbs, finely chopped onion, minced garlic, egg, milk, Worcestershire sauce, Dijon mustard, dried thyme, rosemary, salt, and pepper in a large mixing bowl.

Form Meatballs by rolling the mixture into balls about 1.5 to 2 inches in diameter and placing them on a plate or baking sheet.

Brown Meatballs in a skillet with olive oil, cooking for 5-7 minutes until golden brown on all sides, then transfer to a paper towel-lined plate.

Make Gravy by adding butter, mushrooms, and onion to the same skillet, cooking until softened, then add garlic, thyme, rosemary, and flour, cooking until the roux is golden.

Add Broth by whisking in beef broth, Worcestershire sauce, Dijon mustard, salt, and pepper, then simmer to thicken.

Combine in Slow Cooker by pouring gravy into the slow cooker and adding the browned meatballs, then setting to low to cook for 6-8 hours.

Cook undisturbed in the slow cooker for maximum flavor and tenderness until done.

Finish Gravy by stirring in heavy cream during the last 30 minutes if desired.

Serve your meatballs over mashed potatoes, rice, or egg noodles, garnished with parsley.