Place the haddock in a saucepan with the milk and bay leaf, bring to the boil and continue boiling for 3 minutes. Remove the fish with a slotted spoon, reserving the milk, and divide between 4 individual gratin dishes.
In a separate saucepan, melt the butter, add the flour and cook over a medium heat, stirring, for a few seconds. Remove from the heat and add the reserved milk, a little at a time, stirring well between each addition. Return to the heat, then bring to the boil, stirring constantly, cooking until thickened. Remove from the heat and add the grated Gruyère and mustard.
Pour the sauce over the fish, dividing it evenly between the dishes. Mix together the ingredients for the topping and scatter over the sauce. Place on the top shelf of a preheated oven, 220°C (425°F), Gas Mark 7, for 10 minutes until the topping is golden and the sauce bubbling. Serve with a simple salad.