Preheat the oven to 200°C.
Melt the butter in a large saucepan over a medium heat. Sweat the leek with some seasoning for 8-10 minutes, stirring occasionally until soft and translucent. Remove from the heat, cover and set to one side.
Rub the salmon fillet with the olive oil and season well. Arrange on a lined baking tray and roast for 8-10 minutes until just cooked.
Meanwhile, preheat the grill to hot. Toast the sesame seed bun halves until lightly golden all over. Sprinkle the crumbled Roquefort onto the bottom halves of the buns. Top with a couple of tablespoons of the sweated leeks.
Remove the salmon when ready and flake into chunks. Top the leek with the salmon and season with a little more black pepper. Sit the top halves of the buns on top of the salmon and arrange on serving plates. Serve immediately.