Heat half the oil in large skillet over medium heat. Cook meatballs, in batches for 5 to 7 min., until browned. Transfer to plate.
Add remaining oil to skillet. Cook onion over medium heat 3 to 5 min., until tender and lightly golden. Sprinkle in flour. Cook, stirring, about 1 min. or until flour is no longer dry. Slowly whisk in broth. Continue to whisk and bring to a boil. Stir in Worcestershire sauce, Dijon mustard, nutmeg and meatballs. Reduce heat to medium-low. Cook, stirring, 3 to 5 min. until meatballs are cooked through and sauce thickens. Stir in cream.
Sprinkle with chives and serve over mashed potatoes or egg noodles, if desired.