Combine all poaching liquid ingredients in a medium saucepan. Bring the poaching liquid to a very low simmer and cook 30 minutes. 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 minutes.
For the cucumber sauce:
1 English cucumber, unpeeled, roughly chopped
Fronds from 1 bunch dill (about 1 cup)
1/2 small red onion
2 large lemons, juiced
1/3 cup mayonnaise
2 tablespoons extra-virgin olive oil
1/3 cup yogurt
Salt and freshly ground black pepper
Combine first four ingredients in a blender and puree. Gradually blend in mayonnaise, oil and yogurt and then season, to taste, with salt and pepper.