Peel away the brown skin and fibrous exterior layer of the jicama (a small knife works best for this), then cut in half. Lay each half on its cut side and slice 1/4-inch thick; cut slices in half diagonally. Slice cucumbers lengthwise in half, scoop out the seeds (if there are a lot), and cut each half diagonally into 1/4-inch thick slices. Cut stem and blossom end off oranges, stand oranges on cutting board and, working close to the flesh, cut away the rind and all white pith. Cut oranges in half, then slice each half crosswise into 1/4-inch thick slices.
Mix the jicama, cucumbers, oranges, radishes and lime juice in a large bowl. Let marinate about 20 minutes, then season with salt. Pile the vegetables and fruit onto a serving platter and drizzle with any accumulated juices. Sprinkle liberally with the powdered chile, top with the optional pickled onions and strew with the chopped cilantro. Garnish with the cilantro sprigs and your simple crunchy appetizer is ready to set before your guests.
Variations and Improvisations: This recipe is very flexible: It can be made with just jicama or just cucumber; sliced young raw turnips make an interesting addition, as does sliced raw fennel, apple or Asian pear. Oranges can easily be varied to grapefruit and tangerines, while the powdered chile may be replaced with bottled hot sauce.