To make Honey mustard:
Stir mustard powder with mustard seeds, 3 tbsp cold water, vinegar and salt in a small bowl. Stir 2 tbsp honey into mustard mixture. Store in a glass jar with a tight-fitting lid. Refrigerate overnight before using.
To make Roasted honey-mustard chicken:
Spread 2 tbsp Honey mustard under the skin of 4 chicken legs in a 9 × 13-in. baking dish. Sprinkle with salt and pepper. Bake in centre of a 425F oven, skin-side down, flipping halfway through, until springy when pressed and juices run clear, about 30 min.