Bring large pot of heavily salted water to boil. Add potatoes and cook until fork tender. Cool, put through a potato ricer and reserve.
In a saucepan, combine 1¾ cups water with wine, onion, bouquet garni, lemon juice and bring to a boil. Add the scallops, cover, and simmer on very low heat just until scallops are cooked through (approx 5 minutes). Remove the scallops with a slotted spoon, season, let cool enough to handle and cut into ½-inch pieces.
Add the mushrooms to the scallop poaching liquid and simmer uncovered for 5 minutes. Strain the cooking liquid into a bowl, and reserve the mushrooms.
In a pot, melt 6 tablespoons of butter with one cup of cream over medium heat. Remove from heat, add potatoes and whisk until smooth. Season with salt and pepper to taste.
In a medium saucepan, melt remaining butter. Whisk in flour and cook over medium-high heat until just beginning to brown. Do not let the mixture get dark. Add 2 to 3 tablespoons of the scallop poaching liquid and whisk until smooth.
Preheat oven to high broil.
Over very low heat, whisk in remaining poaching liquid. Add ½ cup cream and simmer, stirring until thickened. Remove from heat, add scallops and mushrooms and season to taste.
Fill six scallop shells (or shallow 6 inch ramekins) with the scallop mixture. Using a piping bag fitted with a #846 star tip, or using a spoon, cover top of scallop mixture with potato. Sprinkle tops with breadcrumbs and grated cheese.
Broil the shells/ramekins in the oven until the mixture bubbles and the cheese melts and turns golden brown. Serve with a sprinkle of chives.