Divide field greens among 4 salad bowls.
Place 1 tablespoon of the oil in medium microwavable bowl. Add walnut halves; stir to coat. Microwave uncovered on High 2 minutes 30 seconds to 3 minutes, stirring every 30 seconds, until walnuts are fragrant.
Stir in remaining 5 tablespoons oil and the vinegar. Microwave uncovered on High about 30 seconds or until dressing is warm but not boiling. Add salt; stir until dressing is well mixed. Pour over salads. Serve immediately.