The Best Leftover Turkey Soup

Name: The Best Leftover Turkey Soup
Ingredients: 2 tablespoons extra virgin olive oil; 2 stalks celery, peeled and sliced; 2 carrots, peeled and sliced; 1 onion, chopped into ¼ inch dice; 1 leek, white part only, cut in half lengthwise and thinly sliced, rinsed in a large bowl to remove grit; 1 bulb fennel, tough outer parts removed, chopped into ¼ inch dice; Kosher salt and freshly ground pepper; 6 cups (1.5 l) turkey stock, bring to a boil before using; 1 pound (0.5 kg) turkey meat, coarsely chopped; 2-3 cups (150-200g) stuffing; Any leftover roasted vegetables, cut into bite sized pieces; Any leftover gravy; ¼ cup (60g) dijon mustard