Preheat your oven to 425°F (220°C).
In a large bowl, whisk together olive oil, Dijon mustard, vinegar, minced garlic and salt. Pat the chicken dry and place the chicken in the marinade, ensuring they are well coated. Marinate for 15-20 mins.
Heat 2 tablespoons of olive oil in a large oven-safe skillet or cast iron pan over medium heat. Remove the chicken from the marinade and shake off any excess. Sear the chicken skin side down until golden brown, about 5-7 minutes. Flip and sear the other side for an additional 3-4 minutes. Remove the chicken from the skillet and set aside. (if the pan is too hot, the honey on the chicken will burn, so watch carefully)
In the same skillet (if there are a lot of burnt pieces in the pan, wash out the pan. If its just golden bits on the bottom, then continue with the recipe as is.), add the sliced onions. Cook over medium heat, stirring occasionally, until the onions are caramelized and golden brown, about 15-20 minutes.
Add the sliced mushrooms to the skillet with the caramelized onions. Cook for another 5-7 minutes, or until the mushrooms are tender and browned. Now add the broccolini pieces and then return the seared chicken thighs to the skillet, nestling them among the veggies.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the chicken is cooked through with an internal temperature of 165F.
Carefully remove the skillet from the oven. If there is a lot of juices in the pan, place the skillet back on the stovetop over high heat. Cook for an additional 5 minutes to reduce the sauce slightly.
Garnish with chopped parsley and serve with quinoa or rice to soak up the delicious juices. Enjoy!