To poach the salmon, place it in a fish kettle or a large roasting tin. Cover with cold water and place the fish kettle or tin over two hobs.
Peel and roughly chop the onion and carrots, then trim and roughly chop the celery. Place in the water with the salmon. Add the bay leaves and the black and pink peppercorns. Cover with a lid or tinfoil and bring to the boil.
Switch off the heat and allow the water and salmon to cool for about 5 hours.
For the pickled red onions, peel and finely slice them, using a mandolin if you have one (use the guard!), then place in a small bowl. Add the red wine vinegar and a pinch of sea salt, scrunch together, then set aside to allow the onions to turn pink.
Meanwhile, make the rhubarb sauce. Trim the rhubarb, peel and core the apple, then chop both into 2cm pieces. Place the fruit in a pan with the honey, bay leaf and whole cloves. Add a splash of vinegar and the orange juice.
Bring to the boil, then turn the heat down to medium. Cover and leave to cook for about 30 minutes, or until the rhubarb has softened. Add a little extra honey to sweeten, if necessary. Tip into a bowl and set aside to cool.
To make the mayonnaise, first make a tarragon oil. Pick the tarragon leaves into a blender, add 100ml of oil and blitz to a pulp. Add the rest of the oil and blitz again, then strain through a piece of muslin into a jug.
Place the egg yolks in a mixing bowl with the mustard and whisk well.
Very slowly drip the tarragon oil into the egg yolk as you whisk; when you’ve added half of the oil, add 1 tablespoon of vinegar, then continue whisking in the oil (you might not need all of it). Finish with a squeeze of lemon juice and a pinch of salt and black pepper.
Carefully remove the salmon to a serving board and gently remove the skin. Break up the salmon flesh into lobes and arrange on a large platter, then rub them with a little oil. Pick the herb leaves over the onions and toss well.
Serve the poached salmon with the pickled onions, some lovely boiled new potatoes and the rhubarb sauce and tarragon mayo on the side.