On Friday night, pianist and lecturer Orli Shaham, making her festival debut, gave a spirited introduction to Music@Menlo’s summer-long celebration of Felix Mendelssohn with the season’s first Encounter, “Forming Mendelssohn”. With the virtuosic assistance of the young artists of the Chamber Music Institute’s International Program, Orli led an in-depth exploration of two masterworks from Mendelssohn’s precocious adolescence: the Piano Quartet in c minor, op. 1, no. 1, and the Double Concerto for violin and piano. Listeners get two more chances to hear the Piano Quartet in full when the festival’s series of Prelude Performances kicks off this weekend. Saturday evening also features the first of three performances of Concert Program I, “From Bach,” featuring pianist and artistic director Wu Han, violinist Arnaud Sussmann, bassoonist Dennis Godburn, San Francisco Symphony principal bassist Scott Pingel, and the St. Lawrence String Quartet (8:00 p.m., St. Mark’s Episcopal Church).