gistfile1.txt

var staffDTOs = ctx.Set<Staff>().ToList().Select(s => new StaffDTO(s));

so_stuff.cs

public class Appointment
    {

        #region Properties

        public int Id { get; set; }
        
        #endregion


        #region Navigation

        public virtual Client Client { get; set; }

        public virtual Staff Staff { get; set; }

        public virtual Service Service { get; set; }

        public virtual Timetable Timetable { get; set; }

        #endregion

    }
    
    public class Staff
    {
        public int Id;
        
        public virtual ICollection<Appointment> StaffAppointments { get; set; }
        
    }
    
    public class AppointmentDTO
    {
      int Id;
      public AppointmentDTO(Appointment entity) {
        this.Id = entity.Id;
        
        // You can copy out more data later if you want
      }
    }
    
    public class StaffDTO
    {
      int Id;
      IReadOnlyCollection<AppointmentDTO> Appointments;
      
      public StaffDTO(Staff entity) {
        this.Id = entity.Id;
        this.Appointments = entity.Appointments.Select(a => new AppointmentDTO(a));
      }
    }