Heat the olive oil in a large frying pan over a medium high heat. Add the leeks and fennel and fry for a few minutes. Lower the heat, cover and cook for 5-8 minutes or until soft but not coloured. Spoon in an ovenproof dish.
Place the salmon on top of the leek & fennel, season lightly with salt and black pepper. Scatter some fennel fronds and bake for 12–15 minutes, or until the fish is just cooked.
While the fish is in the oven mix all the ingredients for the mustard vinaigrette.
Enjoy this dish on its own or accompanied by some rice or potato mash.