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.
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.
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.