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.
Notes
Prep/Cooking time includes marinating time. If using boneless chicken thighs, reduce cooking time - start checking doneness after 25 min.