1) For the salmon: Combine all poaching liquid ingredients in a medium saucepan. Bring the poaching liquid to a very low simmer and cook 30 mins. Skim the top as needed to remove any foam from the surface. Lay some cheesecloth over the top of the poaching liquid, making a bed for the fillets. Submerge salmon in poaching liquid and cook for 8 to 10 mins.
2) For the cucumber sauce: Combine first four ingredients in a blender and puree. Gradually blend in mayonnaise, oil and yoghurt and then season, to taste, with salt and pepper.
3) Serve poached salmon with side of sauce.