Frank Zappa technically “went solo” in 1967, recruiting an army of session players for the orchestral sound-collage lunacy of Lumpy Gravy . But that album—which featured several members of his ragtag backing band The Mothers of Invention— felt more like a one-off detour than a new destination. Two years later, after dissolving the group’s original line-up, the guitarist-composer was, to borrow one of his own titles, absolutely free: Adding his own avant-garde flavor to the burgeoning jazz-fusion aesthetic, Zappa hunkered down in the studio with a rotating crew of hip studio players (including one Mother of Invention, woodwind/keyboard whiz Ian Underwood) and emerged with Hot Rats , the defining triumph of his early career. No longer composing around the Mothers’ technical limitations (they were creative musicians but not exactly virtuosos), the mustachioed mastermind funneled his most grandiose ideas into a newly unveiled six-track console— from the dense, carnivalesque arrangements of “Peaches en Regalia” and “Son of Mr. Green Genes” to the long-grinding blues jams of “The Gumbo Variations” and “Willie the Pimp” (featuring the delightfully demented holler of Captain Beefheart). Five decades later, The Hot Rats Sessions empties the cupboards from those prolific sessions, collecting basic rhythm section tracks (complete with Zappa’s hilarious producer-mode talkback: At one point, he requests that drummer John Guerin “destroy the mood completely”), alternate mixes (including an incendiary, guitarheavy “1969 Mix Outtake” of “It Must Be a Camel”) and rare-tounheard nuggets (the bluesy, violin-dominated instrumental “Bognor Regis”). It’s easy to bask in Hot Rats ’ fluid firepower and forget how much sweat and strain powered these compositions. Zappa was a genius, but he worked hard at it.