1.   Place a large pot half full of water. Bring to a boil. Drop in the green beans, cook until beans are cooked through to an al dente or crisp stage. Drain and transfer to a bowl

2.   Place a skillet over medium heat. Add butter, and when melted drop in the mushroom slices, but do not crowd. Sauté mushrooms to a light golden stage. Scoop and transfer to the bowl with green beans. Add the onions to the skillet; sauté to translucent stage. Scoop and add to bowl

3.   Make the salad dressing. To a small bowl add the olive oil, vinegar, Dijon mustard and salt. Stir well to a homogeneous consistency

4.   Pour the dressing over the green bean mixture. Toss well to coat. Serve either warm or chill in the refrigerator