It's tough, because the Jewish population/presence in Boston is a shadow of it's former self, and compounding the issue is the interest on the part of the current generation to continue, preserve, and maintain this particular style of food.

As a "jew-ish" man who grew up dining at the Titans of Kosher /Jewish eateries in New York, I too lament the dearth of passable Jewish delicatessen -style eateries in my adopted hometown. Mamelahs is certainly what I would suggest to visiting friends and family if they were looking for the experience, and Zaftigs as well, but not to anyone looking for legit Kosher, as neither of them are.

(Unless a newcomer has burst upon the scene, Rubin's of Brookline which closed several years ago, was the only certified Kosher deli in the Boston area)

Kupel's bakery in Brookline is probably the best authentic kosher bakery left in the immediate Boston area, as well as Rosenfeld's Bagel co. in Newton Center, if you were hankering for a bagel vit a nice schmear. There are certainly plenty other good Jewish and jewish-style spots, most of them to be found In Brookline, Newton, Needham, and those other western suburbs where the bulk of Boston's Jewish population drifted to.

this article has a fairly comprehensive list of worthwhile places to eat but investigate before schlepping blind: the article is pre-pandemic and some places might have relocated or closed.