Combine first 7 ingredients in a large zip-top plastic bag. Add mushrooms to bag; seal. Marinate 10 minutes, turning occasionally. Remove mushrooms from bag, reserving marinade.
Sprinkle mushrooms with Cajun seasoning. Heat 2 teaspoons oil in a large nonstick skillet coated with cooking spray over medium-high heat until hot. Add mushrooms; cook 2 minutes on each side or until very brown. Cool; cut mushrooms diagonally into thin slices.
Arrange 4 cups salad greens on each of 4 plates. Top each with mushroom slices, 2 tomato wedges, and onion rings. Sprinkle each with 1/4 cup beans and 1 tablespoon blue cheese. Drizzle the reserved marinade evenly over salads.