Stir the carrots, celery root and 1/2 cup prunes in a 6- to 8-quart slow cooker. Toss the beef with the ras el hanout, 1 teaspoon salt and a few grinds of pepper in a large bowl. Arrange on top of the vegetables. Whisk the chicken broth and tomato paste in a small bowl or liquid measuring cup and pour over the meat and vegetables. Cover and cook on low until the meat is tender, 7 to 8 hours.
About 15 minutes before serving, cook the couscous as the label directs.
Skim the fat off the top of the stew with a ladle and discard. Stir in the harissa and season with salt and pepper. Divide the couscous and stew among bowls. Top with the remaining 1/4 cup prunes and cilantro; serve with more harissa.