In a large mixing bowl, combine ground beef, ground pork, breadcrumbs, finely chopped onion, minced garlic, egg, milk, Worcestershire sauce, Dijon mustard, dried thyme, rosemary, salt, and pepper. Mix gently until combined.

Roll small portions of the meat mixture into uniform balls, about 1.5 to 2 inches in diameter. Place on a plate.

Heat olive oil in a skillet over medium-high heat. Brown the meatballs on all sides for 8-10 minutes. Remove from skillet and set aside.

In the same skillet, melt butter. Add sliced onion and mushrooms, sauté for 5 minutes. Add minced garlic and cook for an additional minute.

Sprinkle flour over the sautéed vegetables, stirring constantly for 1-2 minutes to create a roux.

Gradually whisk in beef broth, scraping up browned bits. Stir in Worcestershire sauce, Dijon mustard, salt, and pepper. Simmer for 3-5 minutes.

Pour the gravy into the slow cooker and gently place the browned meatballs into the sauce, ensuring they're submerged.

Cover and cook on low for 6-8 hours or high for 3-4 hours.

If desired, stir in heavy cream about 30 minutes before serving for a creamy gravy.

Serve the meatballs and gravy over mashed potatoes, rice, or noodles, garnished with fresh parsley.