Place the beef shoulder roast in a large pot. Add the whole onion, garlic, bay leaves, and allspice. Cover with water and simmer until the meat is just tender, about 1½ hours.
Remove the beef and strain the cooking liquid. Reserve the broth. Let the beef cool.
Slice the beef. Heat the oven to 350°F/180°C convection.
Arrange the beef in a large roasting pan. Add the onions, garlic, mushrooms, and chestnuts.
Combine the olive oil, salt, pepper, turmeric, ras el hanout, paprika, date syrup, and hot water. Pour over the beef and vegetables.
Add enough reserved beef broth to cover the ingredients. Sprinkle with thyme.
Bring to a simmer, cover, and cook for 1 hour.
Transfer the covered pan to the oven and cook for 1 hour more. Uncover during the final 15 minutes and cook until the top is browned.
Scatter with fresh thyme and serve with the pan juices.