In a big skillet, heat oil at medium-high heat. Add chicken thighs and brown on each side. Set aside in a plate.
Remove the fat from the skillet. Add mushrooms, onion, garlic, thyme, salt and pepper. Cook and stir for about 8 minutes or until the liquid of mushrooms has evaporate. Sprinkle with flour and cook, while stirring, for about 1 minute. Add milk and bring to boil while scraping the bottom of the skillet to remove the particles.
Put the chicken thighs back in the skillet and their juices. Add dijon mustard. Reduce the heat, cover and let simmer for about 10 minutes or until the chicken is not pink inside. When ready to serve, sprinkle with green onions.