- In a pot of salted water, cook the potatoes until tender. Drain. With a masher, coarsely crush the potatoes with the butter.
- With an electric mixer, purée the potato mixture with the cream until smooth. Add more cream, if needed. Season with salt and pepper. Set aside in a pastry bag fitted with a large star tip.
- With the rack in the middle position, preheat the oven to 350°F (180°C).
- In a pot, soften the shallots in the butter. Add the flour and cook for 1 minute, stirring constantly. Add the milk and wine and bring to a boil while whisking. Cook for 1 minute. Remove from the heat. Season with salt and pepper. Add the scallops and ½ cup (125 ml) of the cheese. Stir to combine.
- Spoon the filling into four scallop shells or four small gratin dishes. Garnish the rim with rosettes of mashed potatoes. Sprinkle with the remaining cheese. Bake for about 10 minutes. Finish under the broiler until the cheese and potatoes are golden brown.