How to Make It
In a medium saucepan, combine 2 tablespoons of the butter with the celery root, cream, water and a pinch of salt. Cover and simmer over moderate heat for 5 minutes. Uncover and cook until the celery root is tender and the liquid is reduced by half, about 10 minutes. Scrape the mixture into a food processor and puree until smooth. Return the puree to the saucepan and season with salt; keep warm.
In a large skillet, melt the remaining 2 tablespoons of butter. Add the chestnuts and cook over moderate heat, stirring occasionally, until lightly browned, about 4 minutes. Add the sliced celery and ras el hanout, season with salt and pepper and cook until the celery is crisp-tender, about 2 minutes. Add the stock and cook until the liquid is nearly evaporated and the vegetables are glazed, about 8 minutes. Keep warm.
In another large skillet, heat the olive oil until shimmering. Season the chicken breasts with salt and pepper. Add them to the skillet and cook over moderate heat, turning once, until golden and cooked through, about 10 minutes.
Spoon the celery root puree onto plates and top with the chestnuts and celery. Arrange the chicken on top, garnish with the chives and tarragon and serve.