comparing left join and outer apply doing the same thing
try this - you might get the best of both  

select manyfield 
from  ( select manyfield
             , row_number() over (partition by t1.id order by t1.timestamp desc) as rownum 
          from inserted 
          left join sometable t1 
                 on inserted.id = t1.id
      ) tt
where rownum = 1 or t1.id is null