In a large bowl, mix the chicken with onion, garlic, salt and pepper. Leave in the fridge to marinate for 6 hours. Add pomegranate paste and mayonnaise. Mix well and put in the fridge to marinate for 2-3 hours.
Preheat oven to 400 degrees F. Pour the melted butter to an 11x8-inch baking dish or larger dish. Transfer the chicken and its marinade to the baking dish. Add the mushrooms and stir gently.
Bake in the oven until chicken is fully cooked through, about 45-55 minutes (cooking time depends on size of the pieces), stirring at halftime. Serve with bread or green salad.