Step 1

Peel and thinly slice 2 medium white onions (about 4 cups). Melt 2 tablespoons unsalted butter in a large saucepan over medium heat. Add the onions and cook, stirring frequently, until the onions have softened but not browned, and most of the liquid has evaporated, about 20 minutes.

Step 2

Add 1 1/2 cups heavy cream and stir to combine. Heat until bubbles just start to form, about 2 minutes. Reduce the heat to medium-low and simmer for 5 minutes. Transfer the mixture to a blender. Start the blender on low speed, then gradually increase the speed to high. Blend until a smooth sauce forms, about 1 minute.

Step 3

Pour the puréed sauce through a fine-mesh strainer set over a small saucepan. Add 1/2 teaspoon kosher salt, 1/4 teaspoon ground white pepper, and 1/8 teaspoon ground nutmeg, and stir to combine. Taste and season with more kosher salt as needed. The soubise will have a texture that's thicker than most sauces but thinner than a purée. Keep warm over gentle heat until ready to serve.