Locate Voodoo Island in Pre-Outbreak mode. To do this, drop into the large black hole in the ground next to the barn, where the alien and some protesters are found. You will reappear on the wooden platform slightly to the left from the way up there. Jump to the adjacent platform and then to the flat rocks to go up from there. Look for the island with a head on it, then travel there. Once on the island, go up to find a giant cauldron with green bubbles. Drag one of the island's inhabitants into that cauldron to get the "Masterchef" achievement.