"These savory and moist turkey meatballs get a little crunch from water chestnuts that will have people asking 'what is that?' The apple-mustard sauce has the perfect balance between savory and sweet."
Preheat oven to 350 degrees F (175 degrees C). Line a baking sheet with aluminum foil.
Mix bread crumbs, egg, 1/4 cup minced onion, water chestnuts, 1/4 cup applesauce, Parmesan cheese, 1 tablespoon parsley, 1/2 teaspoon salt, 1/2 teaspoon mustard powder, and poultry seasoning together in a large bowl. Add ground turkey and gently mix together until just-combined.
Roll turkey mixture into 1 1/2-inch meatballs and place meatballs on prepared baking sheet.
Bake in the preheated oven until cooked through, about 20 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).
Whisk apple jelly, 1 cup applesauce, cider vinegar, 1/2 cup minced onion, ketchup, chili sauce, tomato paste, brown sugar, Dijon mustard, 1 teaspoon dry mustard, and 1/2 teaspoon salt together in a saucepan; bring to a simmer and cook, stirring occasionally, until flavors combine, about 10 minutes.
Transfer cooked meatballs to simmering sauce, cover the saucepan with a lid, and simmer until flavors blend, about 10 minutes. Garnish with remaining parsley.