Spread both types of sesame seeds on a large plate, then dip in the salmon fillets so that the top side of each is evenly coated. Heat the olive oil in a shallow frying pan and fry the salmon over a medium heat for 4 minutes on each side, or until golden and cooked through. Remove the pan from the heat and drizzle the toasted sesame oil over the top.
Halve the rolls and toast under a preheated grill. Top each roll bottom with some cucumber and onion, then add a salmon fillet. Finish with the roll tops and serve immediately with extra cucumber and onion.