To prepare vinaigrette, combine first 6 ingredients; stir with a whisk. Set aside.
To prepare the quesadillas, drain the beans through a sieve over a bowl, reserving 1 tablespoon bean liquid, and discard the remaining liquid. Combine 1/2 cup beans, 1 tablespoon reserved liquid, and oregano in a small bowl, and mash with a fork. Spread bean mixture evenly over 2 tortillas. Sprinkle each tortilla with 1/4 cup cheese; top with remaining tortillas, and set aside.
Sprinkle fish with 1/2 teaspoon pepper. Heat the oil in a large nonstick skillet over medium-high heat. Add fish; cook 2 minutes on each side until fish is medium-rare or desired degree of doneness. Remove fish from pan; coarsely chop fish.
Heat pan coated with cooking spray over medium heat. Cook quesadillas for 4 minutes on each side or until golden brown. Remove quesadillas from pan; cut each into 8 wedges. Combine the vinaigrette, remaining beans, greens, tomato, and olives; toss well. Arrange 2 cups salad on each of 4 plates; divide the fish evenly among salads. Top each serving with 4 quesadilla wedges.