Mustard Beurre Monté

Bring water to simmer in small saucepan over medium-high heat; reduce heat to maintain very gentle simmer. Add 1 piece butter and cook, whisking constantly, until butter is melted, 20 to 30 seconds. Continue to cook, whisking in butter 1 piece at a time until all butter is incorporated and sauce has consistency of thin gravy, 3 to 4 minutes.

Whisk in mustard and season with salt to taste. Reduce heat to lowest possible setting and cover to keep warm. Before using, whisk sauce vigorously to blend in any butterfat that has collected on surface.