- Cook potatoes in a pot of salted water until tender. Drain and transfer to a bowl. Add 30 mL (2 tbsp.) butter, and mash using a hand masher. Add milk and purée with an electric mixer until smooth. Season with salt and pepper.
- Transfer the potato purée into a pastry bag with a large fluted tip. Set aside.
- Place rack in the middle of the oven. Preheat the oven to 180°C (350°F).
- Melt remaining butter in a saucepan. Fry the leeks over low heat for 5 minutes or until softened. Add garlic and cook 2 minutes. Add flour and stir well. Add the wine, stir, and bring to a boil. Add the milk and whisk until sauce is well blended.
- Add 125 mL (½ cup) of grated cheese and the seafood. Season with salt and pepper.
- Place four shells or small gratin dishes on a baking sheet. Divide seafood mixture into them.
- Garnish the outside of shells with the potato purée. Sprinkle with remaining cheese.
- Bake for 10 minutes. Finish cooking under the broiler until cheese and potatoes are golden brown.
If you don't have a pastry bag, use a resealable freezer bag. Place potato purée in the bag and cut off one corner to create a makeshift pastry bag.
It's very important to pat the seafood dry so it doesn't make the sauce too runny.