The meatballs are baked and the onions are caramelized. After adding the flour and broth to the onions, remove from the heat (do not simmer until it thickens or add the vinegar yet). Add the meatballs to the onion mixture, let them cool, then place them in the refrigerator for up to two days. When ready to serve, return it to the pan and bring to a simmer. Continue with recipe as directed.
storage: Leftovers can be refrigerated in an airtight container for up to 2 days.