Heat olive oil in a large Dutch oven or heavy skillet over medium-high heat. Season the chicken thighs with salt and pepper. Add the chicken, skin-side down, and sear for about 5-7 minutes until the skin is golden brown. Flip the chicken and sear for another 5 minutes. Remove the chicken from the pot and set aside.
2. Caramelize the Onions:
In the same pot, add the sliced onions. Cook, stirring occasionally, for about 10-15 minutes until the onions are soft and caramelized. If the onions start to stick, add a splash of chicken broth to deglaze the pan.
3. Add Flavorings:
Once the onions are caramelized, stir in the garlic and cook for about 1 minute until fragrant. Add the Dijon mustard, thyme, and remaining chicken broth and apple cider. Bring the mixture to a simmer.
4. Braise the Chicken:
Return the seared chicken to the pot, skin-side up. Cover and reduce the heat to low. Let the chicken braise for about 30-40 minutes until it’s cooked through and tender (internal temperature should reach 165°F or 74°C). If using, add apple cider vinegar during the last 5 minutes of cooking.
5. Serve:
Remove the pot from heat and let it sit for a few minutes. Serve the chicken topped with caramelized onions and spoon some of the braising liquid over the top. Garnish with fresh parsley if desired.