The flavorful butter adds plenty of zip.
Serves 8 to10.
Using fork, blend butter, mustard and lemon peel in small bowl. Mix in cup chives. Season with salt and pepper.
Cook cauliflower in large pot of boiling salted water 2 minutes. Add broccoli and cook until vegetables are crisp-tender, about 3 minutes longer. Drain. Transfer vegetables to bowl of ice water. Drain. (Butter mixture and vegetables can be made 1 day ahead. Cover separately and chill.)
Combine butter mixture and vegetables in large pot. Toss gently over medium heat until vegetables are heated through and coated with butter mixture, about 5 minutes. Transfer to bowl. Sprinkle with remaining 1 tablespoon chives.