Step 1: Season the chicken thighs generously with salt and pepper on both sides. In a large heavy skillet or Dutch oven, heat olive oil over medium-high heat. Place chicken skin-side down and sear for 5-7 minutes until the skin is deep golden brown and crispy. Flip and sear for another 2-3 minutes. Remove chicken and set aside on a plate.
Step 2: In the same skillet, add the sliced onions and apples. Sauté in the rendered chicken fat for about 5 minutes until they begin to soften and brown. Stir in the minced garlic and cook for 1 minute until fragrant.
Step 3: Stir in the Dijon mustard and then pour in the apple cider and chicken stock. Use a wooden spoon to scrape the browned bits from the bottom of the pan.
Step 4: Place the chicken thighs back into the skillet, skin-side up, making sure they are nestled among the apples and onions but the skin remains above the liquid level. Add rosemary and thyme sprigs.
Step 5: Reduce heat to low, cover, and simmer for 25-30 minutes. Remove the lid for the final 10 minutes of cooking to allow the sauce to reduce and thicken into a glossy glaze.
Step 6: Serve the chicken thighs topped with the braised apples and onions, drizzled with the cider reduction.