Clean and wash the chicken thighs well. Cut them into 2 pieces. I used chicken without skin.
Add salt, pepper powder to the chicken pieces and mix well. Marinate it for about 30 minutes inside fridge.
In a cast iron pan add olive. Add crushed garlic and sauté until it turns brown. Now add the marinated chicken pieces. Roast it until it turns brown on both sides. Remove from the pan and set it aside.
Now add butter and let it melt. Add thin sliced onions to the pan and cook on low flame until the onions caramelize. It would take some time to caramelize the onions. Be patient.
Once the onions are caramelized, add thyme and sauté for a minute.
Add flour, whisk well. Make sure no lumps are formed.
Add chicken stock, continue to whisk and cook until thickened.
Add the roasted chicken into the pan, cook for 5 to 7 minutes.
Finally add vinegar and Dijon mustard mix well and cook for 3 minutes.
Now add grated Gruyere cheese on top of the chicken, broil it for 2 to 3 minutes until cheese melts and bubbles up.
Serve immediately with French bread.