In a bowl, mix together ground beef, breadcrumbs, chopped onion, egg, minced garlic, Worcestershire sauce, Italian seasoning, salt, and pepper until just combined.
Roll mixture into 1-1.5 inch meatballs (yielding about 20-24).
Sear meatballs in olive oil until browned on all sides; set aside.
In the same skillet, sauté sliced onions and mushrooms until softened; add minced garlic.
Sprinkle flour over vegetables and cook for 2 minutes to form a roux.
Slowly whisk in beef broth; add Worcestershire sauce, Dijon mustard, thyme, salt, and pepper. Simmer until thickened.
Layer gravy in slow cooker; add meatballs and cover with remaining gravy.
Cook on low for 6-8 hours or high for 3-4 hours.
For thicker gravy if desired, use cornstarch slurry during the last half hour of cooking.