Boston is pretty poor in the kosher deli area. There are some good ones in the Burbs though, demographics I guess.
Your best bet in the area would be Zaftigs or Rubins on Harvard Ave. in Brookline, both easly accesible from the Green line of the T.
Over in Chelsea just across The Bridge is Katz’s bagle bakery very good bagles. There use to be and may still be there Maxies Deli in Chelsea also real old school type deli, I’ll check on that one. Chow Jim