Cleanest Way To Map Entity To DTO With Linq Select?

Just use AutoMapper.

Example:

Mapper.CreateMap<Address, AddressDTO>();
Mapper.CreateMap<Person, PersonDTO>();

Your query will execute when the mapping is performed but if there are fields in the entity that you’re not interested use Project().To<> which is available both for NHibernate and EntityFramework. It will effectively do a select on the fields specified in the mapping configurations.