1 small head of cauliflower (about 1¾ pounds)
1 t coarse kosher salt
6 T (¾ stick) butter
2 T fresh lemon juice
2 T whole grain Dijon mustard
1½ t finely grated lemon peel
1 T chopped fresh parsley
Preheat oven to 400°. Butter rimmed baking sheet. Cut cauliflower in half, then cut crosswise into ¼-inch-thick slices. Arrange slices in single layer on prepared baking sheet; sprinkle with salt. Roast until cauliflower is slightly softened, about 15 minutes.
Meanwhile, melt butter in small saucepan over medium heat. Whisk in lemon juice, mustard, and lemon peel.
Spoon mustard-lemon butter evenly over cauliflower and roast until crisp-tender, about 10 minutes longer. DO AHEAD: Can be made 2 hours ahead. Let stand at room temperature. If desired, rewarm in 350° oven until heated through, about 10 minutes.
Transfer cauliflower to platter. Sprinkle with parsley and serve warm or at room temperature.