Blanche green beans by placing into a pot of salted boiling water for two minutes. Drain immediately and add green beans to an ice-cold water bath for two minutes. Remove and set aside.

Heat olive oil in a nonstick skillet. Add mushrooms, salt, pepper, and garlic. Cook until mushrooms are slightly browned at the edges. Set aside.

Prepare the dressing. Whisk together olive oil, vinegar, mustard, salt, and pepper.

To assemble, place green beans on a platter. Top neatly with tomatoes and mushrooms. Sprinkle with feta cheese and garnish with scallions. Pour vinaigrette over salad and serve (you can also serve the dressing in a small bowl next to each plate).