Heat a heavy skillet over high heat for 10 minutes. In a small bowl, combine mayonnaise with 1 to 2 teaspoons of Cajun seasoning; taste; add salt, pepper or more Cajun seasoning as desired; set aside.
Lightly oil both sides of salmon fillets and sprinkle with remaining Cajun seasoning (the more you add, the hotter it becomes, so be careful!).
Place salmon in the hot skillet to blacken; cook for 2 to 3 minutes on the first side, then turn and finish cooking. Cooking time will vary, based on thickness of fish and the heat of the pan.
Spread 1 tablespoon of reserved mayonnaise mixture on each French roll; top each with a blackened salmon fillet, 2 slices of tomato and 1/6 of the shredded cabbage; serve.