Place chicken, sumac, olive oil and salt and pepper in a bowl. Coat very well. Heat a non stick frying pan and sear the chicken for 3 minutes on each side or until cooked through. Keep in a warm area.
Place olive oil and butter in a large frying pan on high heat. Sauté the mushrooms for 5 minutes. Now add the garlic, parsley and lemon juice and cook for a further 3 minutes. Season well with salt and pepper.
To serve, place a large mound of mushrooms on each piece of warmed sourdough. Slice the chicken into three, on a diagonal. Place a few slices on top of the mushrooms. Add the slices of baked ricotta and drizzle with the extra virgin olive oil.