If picking a fresh crab, reserve the claw meat to add just before serving.
Put the rice in a pan with the milk and bay leaf. Bring to the boil, lower the heat and simmer until the rice is soft, about 20 minutes.
While it is cooking, sweat the onion in the butter until soft and translucent. Reserve.
Remove the milk from the heat, stir in the crab meat (except for the claw