Place leftover turkey (meat, bones, skin) into a stock pot. Add 1 chopped onion and 2 ribs chopped celery and water or chicken broth (or half of each). Bring to a simmer over high heat; reduce heat to low and continue to cook at a low simmer. Skim off scum as it forms, but don't remove any fat. Cook until broth is rich and any turkey meat has been rendered flavorless, about 3 hours. While broth cooks, skim off fat as it rises to the surface and save it in a bowl. Save about 4 tablespoons turkey fat.

After removing surface fat, strain broth into a bowl.

Lightly whisk 2 eggs in a separate mixing bowl. Add 2 tablespoons cooled rendered turkey fat and whisk to mix. Whisk in salt, pepper, and cayenne. Mix in seltzer water. Add matzo meal and mix with a fork until mixture begins to thicken up, 1 or 2 minutes. Cover bowl with plastic wrap and refrigerate until firm, about 30 minutes.

Heat 2 tablespoons turkey fat in a large pot over medium heat. Add 1 cup diced onions, 1/2 cup diced carrots, 1/2 cup diced celery, and salt. Cook until onions start to turn translucent, 6 or 7 minutes. Add sliced garlic and cook an additional minute. Add turkey broth and bring to a simmer. Simmer until vegetables are tender, about 20 minutes. Check seasoning and adjust salt, if necessary.

Divide matzo ball dough into 8 pieces and roll each into a ball using your wet hands.

Bring a pot of salted cold water to a boil. Gently place matzo balls into the boiling water. Reduce heat so they simmer gently; cover the pot. Simmer until matzo balls are cooked through, about 30 minutes, adjusting heat as they cook so a gentle simmer continues. Transfer to hot serving bowls, 2 per serving.

Stir freshly chopped parsley and dill into soup. Ladle hot soup over the matzo balls.