var staffelpreiseGrouped = from staffelpreis in GetStaffelpreise() // Returns another custom object from a 3rd-party lib -> I can't change that
                                           group staffelpreis by staffelpreis.Materialnummer into g
                                           orderby g.Key descending
                                           select new MyStrongType()
                                           {
                                               ID = g.Key,
                                               Staffeln = g // Staffeln == List<Staffel> -> Staffel.Amount and Staffel.Price are two properties I want to fill
                                           };