Blanch the spinach in boiling water until tender, then drain, squeeze dry and chop.
Add vinegar, salt, sugar and onions to a small saucepot and bring to a boil. When sugar has dissolved, remove from heat, strain and let cool.
Add butter to a large skillet and melt. Add flour and whisk to combine. Cook, whisking, until flour is cooked out and a roux forms, about 2 minutes. Add the cream and bring to a boil until thick, about 2 minutes. Add the onions and spinach and mix until fully incorporated. Continue to cook until thick and creamy, about 5 minutes. 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, wipe the melted edge of the raclette and pour the melted cheese right over the bowl of spinach. Serve!
This recipe has been updated and may differ from what was originally published or broadcast.