Whole fish with the head on, or off such as the salmon used here, must be trimmed, scaled and gutted before cooking. Stovetop poaching affords you most control. A fish kettle is made for this task - the entire fish fits comfortably on a rack in the cooking vessel, with enough room for the poaching liquid to cover it.
Measure the thickest part of the fish. Put the fish on the rack and lower into the kettle. Cover the fish with cold court bouillon. Check seasoning. Bring to the boil.
Reduce to a simmer. Poach the fish for 10 minutes for each 2.5 cm width. Cool the fish in the kettle to retain moisture. Remove and turn out on to baking parchment.