Preheat the oven to 425 Degrees F. Line a baking pan with parchment paper. Set in the salmon. Drizzle fish with oil and lemon juice; season with salt and pepper or Cajun spice. Bake 12 to 15 minutes, or until cooked through. Spread buns with mayonnaise. Top bottom buns with lettuce, and then set on a salmon fillet, removing the skin first, if desired. Divide and top salmon with tomato, cucumber, onion and bell pepper. Set on top buns and serve.