Well, kinda depends on what you’ve got locally. I get frozen clams on the half shell from my LFS. Others get them from their local grocery store. A friend of mine who ironically works at my LFS uses mysis and pellets which haven’t been soaked long enough to get soft to helps manage the beak. I’ve also read that putting that masstick food on shells and rocks being an effective way to keep the beak in check. I’ve been personally using pellets, clams, and frozen mysis and so far it’s been working.
mind you, I’m about 5 months into saltwater so I’m by no means an expert. I’m just sharing what’s worked for me in the 4 months I’ve had my valentini. Ymmv. And I’m sure people more experienced than me will be able to give even better advice!