With a sharp knife, make 2 slashes crosswise into the skin and meat of each chicken thigh cutting to the bone. Cuts should be about 1-inch apart. Transfer thighs into a heavy resealable plastic bag.

In a small bowl, whisk until smooth garlic, Dijon mustard, coconut sugar, red wine vinegar, mustard powder, salt, black pepper, chipotle pepper, and cayenne pepper.

Pour marinade into bag over chicken thighs. 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 in a single layer onto prepared baking sheet. Place chicken thighs on top of onion rings. Spray or brush thighs with 2 tsp olive 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, approximately 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, approximately 3 to 4 minutes. Skim excess fat from pan sauce.

Spoon reduced pan sauce over each chicken thigh and serve with your favorite side dish.