• 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, honey, 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, seal bag, and massage marinade into chicken,coating each thigh thoroughly and working the marinade into the cuts. Refrigerate at least 4 hours (or overnight for best flavor).
• You can also freeze at this point - label bag with date and use by months.
• To cook, 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, adjust onions on top and serve.

Optional

• Pour pan drippings into a saucepan, bring to a boil, and continue boiling, stirring often, until drippings are reduced. Strain in a fat separator (gravy separator) and add juices back to sauce pan and boil for additional 3 to 4 minutes until thickened.
• Spoon reduced pan sauce over each chicken thigh and serve.