PABU Boston, one of the most anticipated openings of the year and the New England debut from star chef Michael Mina, officially opened yesterday inside Downtown’s 60-story Millennium Tower. The chic Japanese restaurant inhabits the second floor of the $700 million project, most of which is dedicated to over 440 luxury residences. Modeled after Mina’s original PABU restaurant in San Francisco, has a clandestine quality, the space is sexy and its upscale izakaya approach — featuring plenty of small plates, an elaborate sushi menu and a massive sake selection — should garner plenty of attention. Here’s what to know before you make your reservation.