Whisk together all listed ingredients before chicken thighs in a small bowl. If you're using xanthan gum (highly recommended), sprinkle evenly over top before whisking (do not add in a clump).
Add chicken thighs to a 7"x11" glass baking dish. Pour mixture on top of chicken and turn chicken to coat it evenly.
Place baking dish in oven on the middle rack. Roast 35 minutes or until internal temp of chicken thighs reaches 165 F. Chicken should be nicely browned on top.
Remove chicken from oven and cover pan with aluminum foil. Allow chicken to rest for 10 mins covered, then season with fresh rosemary and additional salt and pepper to taste.
Serve with some of the pan sauce spooned over top.
Adjust dijon mustard based on your preferences. In my opinion, 4 tbsp dijon gives this recipe the perfect sweet-tangy mix. However, if you aren't a big mustard fan, try using only 3 tbsp for a sweeter chicken without the hint of mustard.