How to Make It
In a small saucepan, melt the butter. Add the flour and cook over moderate heat, stirring, until deep golden brown, about 5 minutes. Add the stock, demiglace, sage and thyme and cook until thickened to a gravy-like consistency, 15 to 20 minutes. Stir in the lemon juice and heavy cream and season with salt and black pepper. Keep warm.
Cut one 1/2-inch-wide strip off the top of each baked potato and reserve. Scoop the potato flesh into a large bowl. Place the potato shells on a baking sheet. Using a ricer, mash the potato flesh with the butter into another large bowl; add the warm milk and mix until blended. Stir in the sour cream, Parmigiano, mustard, cayenne and chives and season with salt and pepper.
In a small saucepan, heat the vegetable oil to 350º. Scrape the flesh off the reserved 1/2-inch strips of the potato tops. Cut the skins into wedges and fry until golden and crisp, about 2 minutes. Drain the skins on a paper towel-lined plate.
Preheat the oven to 450º. In a small nonstick skillet, cook the bacon over moderate heat, stirring occasionally, until golden and crisp, 5 to 7 minutes. Drain the bacon on paper towels.
Spoon about 2/3 cup of the mashed potato mixture into each potato shell and make a well in the center. Bake for about 20 minutes, until heated through and golden on top. Spoon some gravy into the well of each potato, then top with the shredded mozzarella. Bake the potatoes for about 5 minutes, until the cheese is melted. Transfer the potatoes to a serving platter and top with the bacon. Garnish with sour cream, scallions, parsley and celery leaves and the crispy potato skins.