In a large bowl, combine ground beef, breadcrumbs, eggs, onion, garlic, Worcestershire, salt, pepper, and parsley. Mix gently.
Roll into golf ball-sized meatballs (about 25–30).
Lightly brown the meatballs in a skillet with oil (2 minutes per side).
Place meatballs in crockpot.
Mix sauce ingredients as above and pour over meatballs. Add butter slices and peppers.
Cook on LOW for 4 hours. Serve with sauce spooned on top.