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.
- 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 45 mL (3 tbsp.) milk. With a electric beaters, purée potatoes 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 potato purée. Sprinkle with remaining cheese.
- Bake for 10 minutes. Finish cooking under the broiler until cheese and potatoes are golden brown.
It's very important to pat the seafood dry so it doesn't make the sauce too runny.