Quarter chayotes and set on a steamer rack over boiling water; cover and cook until not quite tender—about 20-25 minutes. Set corn on rack during the last 7-8 minutes. Let both cool briefly. Pull off all peel from the chayotes and cut flesh into ½-inch cubes.
Cut corn kernels from the cobs; combine in a bowl with chayotes, tomatoes, and scallions. Blend togethe