Early this morning, 2 a.m. to be precise, Eric Greenspan issued a hamburger challenge to LA’s reigning hamburger king, Sang Yoon of Father’s Office. Greenspan, chef-owner of The Foundry on Melrose, staged what can only be called a covert food op, nailing his challenge—with ketchup—to the doors of the two Father’s Office locations, in Culver City and Santa Monica, under a thick cover of darkness and muffled hilarity.