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 of the oven. Preheat 450. 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.
TIPS: Always leave skin on chicken when roasting or it will dry out. Free-range chicken has much less fat than store-bought chicken raised in a confinement house so you don't have to worry about a lot of excess fat.