Step 1
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.
Step 2
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.
Step 3
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).
Step 4
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.
Step 5
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.
Step 6
Roast chicken in preheated oven until the skin is browned meat is tender and the juices run clear 35 to 45 minutes.
Step 7
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.
Step 8
Spoon reduced pan sauce over each chicken thigh and serve.