Place the salmon in a fish kettle. If you haven't got one a large roasting tin will do fine.
Add enough cold water to cover the fish (it must be covered). Add the onion, bay leaves, vinegar and lemon and a good pinch of salt and a little black pepper.
Cover with a lid, or tin foil, and bring to the boil on the stove. It is easier and quicker to put two rings on underneath the fish. Once it is boiling, turn off the heat and allow the fish to stand in the water until cool.
Carefully remove the salmon and place onto a board. Carefully scrape off the skin and place on a large flat serving dish. Serve with a bowl of lemon wedges and a bowl of homemade mayonnaise and steamed vegetables.
Put the yolks in a bowl (blender, food processor) with the mustard, if using and 1 tsp lemon juice and mix those ingredients together. Start whisking vigorously (blender or food processor on low) while dripping the oil very slowly, even drop by drop in the beginning. You're creating an emulsion and if you put too much oil at once, it will separate and will be very hard to save. Whisk non-stop and use a towel under the bowl to help stabilize it. As you add more oil, the emulsion will form and the mayonnaise will start to thicken and you can pour the oil faster at this point. When all the oil is incorporated and the mayonnaise is thick, add the rest of the lemon juice and taste your creation. You can season to taste with salt and pepper.