Soubise (French Onion Sauce)

In a medium saucepan, melt 2 tablespoons butter over medium heat until foaming. Add onions along with a large pinch of salt and cook, stirring frequently, until softened and most liquid has evaporated, about 20 minutes. Stir in cream and simmer for 5 minutes.

Transfer hot onions and cream to a blender. Add curry powder or Vadouvan if using, and blend, starting at low speed and gradually increasing to high, until a smooth sauce forms. If desired, pass sauce through a fine-mesh strainer, pressing on solids with a spoon, for an even more refined texture. Return onion sauce to saucepan and season with salt and white pepper (note that sauce should have a texture that's thicker than most sauces but thinner than a puree). Keep warm.