Preheat the oven to 375. In a 12-inch oven-safe frying pan, add slab bacon and render fat stirring until lightly brown, remove from pan set aside until later, add butter and continue to brown over medium high heat. Season chicken thighs with salt & pepper.
Add thighs skin side down, working in batches cook 3 each for about 2 minutes per side, until the chicken is well browned. Do not over crowd. Flip over the chicken and sear the other side. Remove the seared chicken and set aside on a plate, then repeat the process with the second batch. Meanwhile, cut the onion in half and slice into 1/4 inch half-moons. Peel the garlic clove.
After the chicken is finished searing, add the onion, reduce the heat to medium and saute until soft. Add the garlic and cider. Bring to a simmer while using a wooden spoon to scrap off the bits on the bottom of the pan. Place the chicken over the onions, pour in the chicken stock, bring to a simmer. Put the pan in the oven, and cook for 40 minutes, basting once or twice.
Remove pan from oven and take out the chicken thighs, then add the quartered apples & rosemary sprigs on top of the onions, then place the thighs on top of the apples. Place back into the oven for another 40 minutes. The chicken is done when a fork or sharp knife is easily inserted into it and the chicken slides off the fork.
The sauce should be reduced by the time the chicken is cooked; set the chicken aside and cover with foil to rest while you season the sauce to taste. Divide the chicken equally between three plates and spoon the sauce over each serving.