Preheat the oven to 200°C, fan 180°C, gas 6. Use a little butter to grease 4 x 500ml ovenproof dishes.
Place the potatoes in a pan of cold water. Bring to the boil, then simmer for 10–15 minutes, until tender. Drain and mash with 25ml milk and 25g butter until smooth and creamy.
Meanwhile, melt 30g butter in a large pan, then add the mushrooms, spring onions and bay leaf, and fry until soft and golden. Stir in the flour and cook for 1 minute.
Pour in the sherry, then gradually add the remaining 175ml milk, stirring until smooth. Season with salt and freshly ground black pepper. Add the scallops and cook for 3 minutes. Remove from the heat, take out the bay leaf and spoon into the dishes.
Top the scallop mixture with the mashed potato and rough up the surface with a fork. Dot with the remaining 20g butter and sprinkle with freshly ground black pepper. Place on a baking tray and cook for 30 minutes, until golden.