Grill with a nutty topping - crush brazil nuts into some chopped breadcrumbs, lemon zest, chives and mustrd or horseradish. Press the coating down to the flesh of the fish and bake in a hot oven - 10 mins should do it.
Serve with a lemon butter. mash and veg.
Or for a suace alone to go with plain grilled, try a cider sauce with apples.
Rhubarb sauce (traditional one this) - colour some onions, add chopped rhubarb, season and cook 5 mins. Pass through a fine seive and serve.
Do the same with gooseberries too...
Try a red pepper marmalade.onion chutney or a balsamic relish - the point is the acidic sauce will balance the oil of the fish.
Have fun!