On April 22, a set of Claude Lalanne’s gilt-bronze mirrors commissioned by Yves Saint Laurent and Pierre Bergé is expected to fetch between $10 and $15 million at auction at Sotheby’s. Created by the French sculptor between 1974 and 1985, the mirrors lined the walls of the couple’s legendary Salon de Musique in their Paris apartment, each one framed by electroplated copper hosta leaves drawn from the artist’s own garden. What began as a commission for two mirrors grew over the following decade into an ensemble of 15, transforming the room into one of the most remarkable decorative commissions of the 20th century. “The mirrors are the masterpiece of Claude Lalanne,” say collectors Jean and Terry de Gunzburg, who acquired the set in 2009.…