Blanch the spinach in boiling water until tender, then drain, squeeze dry and chop. Bring a large pot of salted water to a boil. Add the spinach to the boiling water and cook until tender, then drain. When the spinach is cool enough to handle, squeeze dry and chop. Set aside.
Meanwhile, combine the vinegar, 1 tablespoon of salt, sugar and onions in a small saucepan and bring to a boil, stirring to dissolve the sugar. Remove from the heat, drain the onions with a slotted spoon and let cool on a plate.
Meanwhile, melt the butter in a large skillet over medium-low heat. Add the flour and whisk to combine. Cook, whisking, until the flour is cooked out and a roux forms, about 2 minutes. Add the cream and bring to a boil, whisking, until thick, about 2 minutes. Add the pickled onions and spinach and stir until fully incorporated. Continue to cook until the mixture is thick and creamy, about 5 minutes. Stir in up to 1 cup milk, as necessary, if the sauce is too thick. Season with salt and pepper. Transfer to a serving bowl.
Meanwhile, using a kitchen torch or broiler, melt the flat, cut edge of the raclette until golden and bubbly. Using a small metal spatula or spoon, scrape the melted edge of the raclette and pour the melted cheese right over the bowl of creamed spinach. Serve immediately!
This recipe has been updated and may differ from what was originally published or broadcast.