In a medium sized saucepan, place potatoes and cover with water. Add salt, bring to a boil, adjust heat to maintain a low boil, and cook until potatoes are tender. Drain, then mash with milk and melted butter until smooth; set aside.

Prepare the bechamel sauce:
In a small saucepan melt the butter over medium heat. Add the flour and cook, stirring, until flour just begins to color and no longer smells raw. Gradually whisk in the milk and simmer until sauce has thickened.

Prepare the seafood filling:
Meanwhile, rinse the shrimp and scallops. Drain well and pat dry.
In a large skillet, heat olive oil on medium heat. Add onions and sauté until softened, 8 to 10 minutes. Add mushrooms and cook until they have released their water and it has evaporated, about 5 minutes.
Add shrimp and scallops and cook, stirring, for 3 minutes. Add salt and pepper to taste. Pour in white wine and cook until it has evaporated. At this point, drain the seafood if there is a lot of liquid in the pan. Tranfer seafood to a bowl.

Pour the sauce over the seafood and stir to combine. Stir in chopped parsley.

To Assemble:

Preheat oven at 350°F. Divide the seafood among 4 scallop shells or ramekins placed on a baking sheet.
Place the mashed potatoes in a pastry bag with star tip or in a freezer bag with the tip snipped off and pipe the potatoes around the seafood filling.
Top each shell with shredded cheese. Bake for about 20 minutes until the cheese is bubbling. If desired, at the end of cooking time broil for a minute or two to brown the cheese. Serve immediately.