In a pot of salted water, cook the potatoes until tender. Drain. With a masher, coarsely crush the potatoes with the butter.
With small stick blender, purée the potato mixture with the cream until smooth. Add more cream, if needed. Season with salt. 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, Sauté the shallots (or red onion) in the butter. Add the flour and cook for a minute or two, stirring constantly. Add the milk (ideally warmed) 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, chives and the cheese. Stir to combine.
Lift 2-3 scallops into each shell and cover with the sauce, pipe the mash around the side of the shell. Sprinkle over dried breadcrumbs.
Bake in the oven for 10-12 minutes or until cooked and golden.