2 tablespoons unsalted butter
  2 medium onions, thinly sliced
  Kosher salt
  1 1/2 cups heavy cream
  1 teaspoon curry powder or Vadouvan spice, optional (see note)
  Ground white pepper, to taste

1. 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.
2. A two-image collage. The top image shows a stainless steel saucepan holding lightly golden soft onions, and the bottom image shows the onions still in the pan with heavy cream being poured in.
3. 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).