Heat oil in a large nonstick skillet over medium-high heat. Add onion; cook 5 minutes, stirring frequently. Continue cooking 15 to 20 minutes or until deep golden brown; stir frequently. Add balsamic vinegar; cook 2 minutes. Remove from heat. Add tomatoes.
Cook haricots verts in boiling water 2 minutes or until crisp-tender; drain.
Combine haricots verts, onion mixture, salt, and pepper in a large bowl. Serve immediately.