Blanch green beans in large pot of boiling salted water until just crisp-tender, about 2 minutes. Drain. Rinse with cold water to stop cooking; drain well. Pat green beans dry. Transfer to large bowl. Add mushrooms and endive. Mix white wine vinegar and Dijon mustard in small bowl. Gradually mix in olive oil. Mix in sliced chives. (Can be prepared 6 hours ahead. Cover vegetables and refrigerate. Let dressing stand at room temperature.) Pour dressing over vegetables and toss to coat. Season with salt and pepper. Serves 6. Bon Appetit January 1994