Start by finely dicing the shallot and leek. Place them into a pan and gently sweat in the butter without colouring. This should take about 15 minutes.
While waiting, cook the fish in simmering milk for 10 minutes and allow to cool slightly. Dice the potato as small and as finely as possible (as this will replace the rice in the dish). Add the crushed garlic and thyme to the onion mix. Then add the potato and stir thoroughly to ensure all is covered in the butter mix.
Start adding the milk one ladle at a time, stirring continuously to allow the potato to absorb the milk. The milk should get quite thick just before the potato is fully cooked. At this point, flake the cooked fish into the potato; add the créme fraîche and season with chervil garnish.