Yes, $21 for two or three pieces of sashimi is pricey. But less is far more at O Ya, where chef Tim Cushman’s top-tier ingredients and meticulous layering of flavors add up to the best mouthfuls in New England. Perfect bites, like fatty Scottish salmon belly with cilantro, ginger, and hot sesame oil, are presented with a careful balance of richness and bracing acid. So yeah, it's costly. But anything else that trips your pleasure centers like this will also cost a fortune — and probably isn’t legal.