Jicama is a root vegetable with a fresh, somewhat sweet taste that goes well with citrus fruits.
Slice 1 orange in half, and squeeze the juice from one half into a bowl. Add sherry vinegar, salt, and pepper; slowly whisk in oil.
Cut away peel and pith of remaining 2 1/2 oranges. Slice oranges into 3/8-inch rounds, and use your hands to pull slices apart into bite-size pieces. Set aside in a large bowl.
Cut jicama into 2-inch matchsticks. Add jicama and onions to orange pieces. Pour vinaigrette over; toss to combine.