Place chicken, potatoes and onions in a large baking dish.
Season with sumac, salt and white pepper.
In a bowl, stir together garlic, olive oil, and lemon juice. Pour over chicken mixture.
Sprinkle evenly with the parsley.
Cover dish with foil.
Bake in preheated oven for 35-40 minutes. Remove foil, increase heat to 450 degrees and cook until chicken and potatoes are golden, about another 30-40 minutes (Depends on how large you cut the potatoes and chicken.).