Toss beef with salt and allspice. Place in slow cooker with carrots, onion and garlic. In a bowl, whisk tomatoes and harissa. Pour over beef. Cook on HIGH for 5 hours or LOW for 7 hours.
To serve, scoop 3/4 cup couscous into each bowl. Spoon stew on top; garnish with 1 tbsp hummus and, if using, parsley.