• Make-Ahead

Ingredients

1. 2 tablespoons yellow mustard seeds
2. 4 medium shallots, thinly sliced
3. 2/3 cup sherry vinegar
4. 1/4 cup water
5. 1/2 cup heavy cream
6. 1 stick (4 ounces) unsalted butter, softened
7. 1/4 cup whole-grain mustard
8. Salt and freshly ground pepper
9. 4 1/2 pounds green beans, trimmed

Directions

1. In a large skillet, toast the mustard seeds over moderately high heat, shaking the pan frequently, until they pop and start to brown, about 30 seconds. Transfer to a plate and let cool. 
2. Add the shallots, sherry vinegar and water to the skillet and simmer over moderately low heat until the liquid is reduced to 1 tablespoon, about 12 minutes. Add the cream and simmer over moderate heat until thickened, about 5 minutes. Transfer to a bowl and let cool to room temperature, then stir in the butter, mustard and toasted mustard seeds. Season with salt and pepper. 
3. In a large pot of boiling salted water, cook the green beans until crisp-tender, about 8 minutes. Drain the beans, pat dry and transfer to a large bowl. Add the mustard-seed butter, season with salt and pepper; toss and serve.

Make Ahead

The mustard-seed butter can be refrigerated for up to 3 days or frozen for up to 1 month. Let the butter return to room temperature before using. The cooked green beans can be refrigerated overnight; blanch in boiling water or steam until heated through.