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