To begin, peel the onions and boil them whole in a pan of water for 20 minutes. Preheat the oven to 180°C/Gas mark 4.
When the onions are tender, take them out of the water and cut them in half. Place them in a heatproof shallow dish, season them, add the cream and sprinkle the grated cheese on top. Put them in the oven for 25-30 minutes.
When the onions have been cooking for 15 minutes, wash and season the salmon. Pour a generous slick of oil on each fillet and squeeze the juice of half a lemon over each. Heat a grill pan and sear the salmon for about five minutes on each side.
By now the onions should be bubbling and golden brown on top. Serve on a plate with the salmon and a plain green salad, with a sharp vinaigrette.