In the grim pandemic nadir of September 2020, violinist Soovin Kim stepped out on an empty stage at the New England Conservatory in Boston and performed Johann Sebastian Bach's Sonatas and Partitas for Solo Violin. These six towering works from the late 1710s — three sonatas and three partitas — take two and a half hours to play straight through and encompass an encyclopedic range of technique and emotion. Chamber Music Northwest — the Portland, Ore., series of which Kim is co-artistic director — streamed a video recording of the concert to homebound audiences as a "gift." That remarkable gift is still available on YouTube, but a greater one is imminent. This weekend, Boston-based Kim, 48, will perform the Sonatas and Partitas at two concerts, one on Friday night and one on Sunday afternoon, at the Cathedral Church of St. Paul in Burlington. The concerts are an off-season offering from the summertime Lake Champlain Chamber Music Festival in Colchester, which Kim founded and co-artistic directs. They give locals a rare chance to see one of the country's most accomplished violinists perform a set of works that the late violinist George Enescu called "the Himalayas" for violinists. "Not many people do solo Bach concerts," Jody Woos said. As executive director of LCCMF from 2013 to 2021, Woos recalled organizing many fundraising house concerts in which Kim played movements of a sonata or partita. Performing them all, however, is "kind of a marathon," she said. "Just to stand and play one of them is an accomplishment," said cellist Edward Arron, Kim's friend of 30 years, who regularly plays at the LCCMF and Capital City Concerts in Montpelier. "Each partita and sonata is a monument. For someone to play all six of those in two days — that is an incredible feat of violin virtuosity, of physical and mental stamina." Each sonata has four movements that alternate between slow and fast. The second movement of each is a fugue, in which one phrase is taken up successively by distinct voices at set intervals — something that might seem inconceivable on one instrument. The violin is naturally single-voiced, but in Bach's hands it becomes polyphonic, Kim explained. The partitas are generally lighter explorations of dance-music forms, each with four to seven movements that range from stately sarabandes to minuets to lively jigs, aka gigues. The exception is Partita No. 2 in D…