Make the glaze - In a bowl, mix Dijon mustard, whole-grain mustard, honey or maple, vinegar, olive oil or butter, lemon juice, thyme, garlic, and cayenne if using.
22 Start roasting skin-side up - Place chicken thighs on a rack set over a baking sheet, or directly on a foil-lined sheet pan. Roast at 425°F for 20 minutes.
23 Glaze the chicken - Brush the mustard glaze generously over the thighs. Return to the oven.
24 Finish roasting - Roast another 15–20 minutes, brushing with more glaze once more if desired, until the chicken is browned and the thickest part reaches at least 175°F. Thighs are especially good closer to 180–185°F, where they get more tender.
25 Rest and serve - Rest 5–10 minutes before serving. Spoon any pan juices over the top.