MassOpera is set to present the world premiere of Eric Sawyer and Ron Bashford’s “The Onion” this September. The opera, which will get its premiere on Sept. 11-13, 2025 at Holden Experimental Theatre and on Sept. 25 and 26 at Huntington Theatre, takes place on an island in the Pacific Northwest and explores the story of a neuroscientist who sequesters {…}