private void SetInsertMetadata(TDomainEntity entity)
 {
      entity.InsertDT = DateTime.UtcNow;
 }

public class GenericClass<TDomainEntity>
{
    ...
}

public interface IDateTimeHolder
{
    DateTime InsertTD { get; set; }
}

public class GenericClass<TDomainEntity> where TDomainEntity : IDateTimeHolder
{
    string Version { get; set; }
}