Prepare the Béchamel sauce: melt a half tablespoons butter in a small saucepan over medium heat. Stir in the flour and cook for a few minutes without allowing it to color. Whisk in the milk. Continue whisking until the sauce thickens. Set aside until needed.
Blanch the onions in boiling, salted water for 15 minutes. Rinse well and drain thoroughly. Place in a saucepan along with half the butter. Cook over low heat until the onions are very soft and a bit dry. Do not let the onions color.
Mix in the Béchamel sauce. Season with salt, pepper, nutmeg, and paprika. Continue cooking over low heat for an additional 10 minutes. 4. Just before serving, purée the mixture with a stick blender. Stir in the remaining butter and a spoonful of crème fraîche.