Add all ingredients for the dressing to a small bowl or jar. Whisk or shake with the lid on until well combined and smooth, about 10 seconds.
Make the salmon: Heat the oven to a high broil. Rub each filet with a spoonful of oil. Sprinkle each filet with about 2 tsp cajun seasoning, then rub all over the salmon. Place on a baking sheet and broil in the oven for 8-11 minutes, depending on the thickness of the filets, until cooked through. Remove and let cool slightly.
Assemble the salad: Toss the lettuce with desired amount of salad dressing. Place a bed of lettuce in 4 separate serving bowls. Top each salad with the black beans, corn, and tomatoes. Sprinkle with green onion. Top with the cooked salmon filets and drizzle with desired amount of BBQ sauce!