- Make 2 slashes crosswise into the skin and meat of each chicken thigh with a sharp knife, cutting to the bone. Cuts should be about 1 inch apart. Transfer thighs into a heavy resealable plastic bag.
- Whisk Dijon mustard, brown sugar, red wine vinegar, mustard powder, salt, black pepper, ground chipotle pepper, and cayenne pepper in a bowl until smooth. Whisk garlic into marinade.
- Pour marinade into bag over chicken thighs and massage marinade into chicken, coating each thigh thoroughly and working the marinade into the cuts. Seal bag and refrigerate at least 4 hours (or overnight for best flavor).
- Move a rack to the center position in oven. Preheat oven to 450 degrees F (230 degrees C). Line a baking sheet with aluminum foil and lightly oil the foil.
- Scatter onion rings onto prepared baking sheet. Place chicken thighs on top of onion rings. Spray or brush thighs with vegetable oil; sprinkle thighs with additional salt and cayenne pepper if desired.
- Roast chicken in preheated oven until the skin is browned, meat is tender, and the juices run clear, 35 to 45 minutes.
- Transfer chicken and onions onto a serving platter. Pour pan drippings into a saucepan, bring to a boil, and continue boiling, stirring often, until drippings are reduced by half, 3 to 4 minutes. Skim excess fat from pan sauce.
- Spoon reduced pan sauce over each chicken thigh and serve.