How to Make It
In a medium saucepan, melt the butter. Add the flour and cook over moderately low heat, whisking, until light golden, about 3 minutes. Whisk in the milk and bring to a simmer, whisking frequently. Cook, whisking, until the sauce is thick enough to coat the back of a spoon, about 5 minutes. Scrape the béchamel into a medium bowl, whisk in the mustard and season with salt. Let cool completely.
Preheat the oven to 350°. In an 8-by-11-inch baking dish, toss the potatoes with the cream, rosemary, 2 teaspoons of salt and 1/4 teaspoon of pepper; mix well. Cover and bake until the potatoes are tender when pierced, about 30 minutes. Uncover and bake until the cream reduces and thickens, about 15 minutes.
Spoon the béchamel over the potatoes and bake for about 10 minutes, until golden and bubbly. Scatter the raclette cheese on top and bake for 5 minutes longer, until melted. Let stand for 10 minutes, then top with the cornichons and serve hot.