Prepare the marinade by combining the spicy brown mustard, sugar, maple syrup, yellow mustard, apple cider vinegar, soy sauce, onion powder, and minced garlic in a bowl; mix until well combined.
Pour half of the mixture into a Ziplock plastic bag, then add the chicken thighs to the bag and squish the chicken around in the marinade until evenly coated. Reserve the remaining marinade (sauce) in a small bowl. Chill for 4 hours.
Cook the chicken by heating the olive oil in an OVENPROOF large skillet over medium-high heat.
Season both sides of the chicken thighs with sea salt and freshly cracked pepper, to taste.
Place the chicken in the hot skillet and cook for 4 minutes or until golden brown. Flip the chicken over and place it into the preheated oven.
Roast for 15-20 minutes or until chicken is cooked through, using a meat thermometer.
Serve with warmed-up reserved mustard sauce. Enjoy.