In a large skillet, heat the vegetable oil over medium heat. Add the finely chopped onion and sauté until soft and golden brown.
Add the minced garlic and cook for another minute.
Stir in the ground walnuts, pomegranate molasses, dried sour plums or apricots, ground sumac, ground coriander, ground cinnamon, ground black pepper, and salt. Cook for a few minutes until the mixture is well combined and heated through. Remove from heat and let it cool slightly.
Preheat your oven to 375°F (190°C).
If using chicken, rinse and pat dry. Season the inside and outside with salt and pepper.
If using fish, clean and scale the fish, then rinse and pat dry. Season the inside and outside with salt and pepper.
Fill the cavity of the chicken or fish with the prepared stuffing, packing it in firmly.
If using chicken, tie the legs together with kitchen twine to keep the stuffing inside. If using fish, you can secure the cavity with toothpicks.
Place the stuffed chicken or fish in a roasting pan. Brush the outside with vegetable oil.
Roast in the preheated oven for about 1.5 to 2 hours for chicken, or about 30-40 minutes for fish, or until the meat is cooked through and the skin is golden brown and crispy. The internal temperature of the chicken should reach 165°F (74°C).
Remove the lavangi from the oven and let it rest for a few minutes before carving.
Enjoy your meal!