Oil grill rack and preheat grill to high.
Rub salmon on both sides with your seasoning of choice (cajun or blackening).
Grill until cooked through, 3 to 4 minutes per side.
Mash avocado and mayonnaise in a small bowl.
To assemble sandwiches, spread some of the avocado mixture on each roll and top with salmon, arugula, tomato and onion.