Bring large pot of water to boiling. Add green beans; cook just until tender, about 5 minutes. Drain; rinse under cold water; drain.
In food processor, combine vinegar, onion, mustard, salt and pepper. Process until onion is finely chopped. With processor running, slowly add oil until dressing is thick and well blended (emulsified).
In large bowl, toss green beans, sweet peppers and jicama with dressing to thoroughly coat. (Can be prepared 3 hours before serving.) Serve cold or at room temperature.