Make ahead: Bake the meatballs and caramelize the onions. After adding the flour and broth to the onions, remove from the heat (do not simmer to thicken or add vinegar yet). Add the meatballs to the onion mixture, cool, and refrigerate for up to 2 days. When ready to serve, transfer back to the skillet, and bring to a simmer. Continue with the recipe as directed.
Storage: Leftovers can be refrigerated in an airtight container for up to 2 days.
