Place the water, sherry vinegar, spring onion, carrot, celery, and thyme in a pot and bring to a simmer. Simmer for 10 minutes. Season the trout on the outside and in the belly with salt, pepper and a pinch of cayenne. Place in the simmering court bouillon and poach for 7 minutes, or until a skewer inserted in the thickest part of the flesh comes out warm.
Remove the fish with a slotted spoon and set aside on a warm platter. Strain the cooking liquid and return 1/2 cup to the pot. Bring the court bouillon to a boil and add the butter slowly, 1 tablespoon at a time, whisking constantly. Continue whisking until all the butter is absorbed. Season, to taste, with salt and pepper.
To serve, place a trout on each plate and spoon the sauce over each. Serve immediately.