How to Make It
In a small dry skillet, toast the mustard seeds over low heat until fragrant, about 5 minutes.
In a large saucepan, combine the butter, horseradish, garlic, shallots, vinegar, honey and mustard with 2 cups of water. Bring to a simmer over moderately high heat and cook over moderate heat, stirring occasionally, until reduced by half, about 10 minutes. Add the quartered radishes and cook until beginning to soften, about 3 minutes. In batches, add the radish tops and mustard greens and cook, stirring, until wilted, about 2 minutes. Season with salt and pepper. Transfer to a shallow serving bowl and top with the sliced radish. Sprinkle with the chives and mustard seeds and serve.