Preheat the oven to 220°C. Peel and slice the ginger and put into a food processor. Put some sprigs of coriander aside and add the rest to the ginger. Cut 1 lime into wedges for a garnish. Grate the rind of the other, squeeze the juice, and add to the mixing bowl with the butter, salt, pepper and cayenne, then blend.
Shape the butter into a sausage, wrap in foil and chill in the freezer.
In the meantime, brush a baking tray with a little oil, place the salmon fillets on it and brush with the remaining oil. Sprinkle with salt and bake for 8 minutes, or until the flesh flakes easily.
Take the butter from the freezer and cut into thick rounds. Transfer the fish to four plates, place a round of butter on each and garnish with the lime and coriander.
Serve with couscous, which you can leave to soak while you cook the salmon.
You can also use rainbow trout or ocean trout in this recipe.