Combine olive oil, vinegar, Dijon mustard, sugar, salt, and pepper in a large bowl. Pierce potatoes with a fork. Place potatoes in a microwave-safe dish; cover loosely with plastic wrap. Microwave at HIGH 5 1/2 minutes or until tender. Cool slightly. Quarter potatoes, and add to vinegar mixture; toss to coat. Heat haricots verts according to package directions. Add haricots verts, green onions, and tarragon to potato mixture; toss.
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