Heat olive oil in a Dutch oven over medium-high heat until hot. Sear each pork loin for 2-3 minutes per side until golden brown. Transfer to a plate. Over medium heat, deglaze the Dutch oven with cider and Cognac, scraping up all the browned bits. Stir in the brown sugar, and continue cooking, stirring occasionally, until the liquid has reduced by half, about 10 minutes. Stir in the onion and the leek, and simmer 5 minutes. Fold in the apple and sprinkle with salt and pepper. Add the thyme, then place the pork loins on top. Cover and cook for 20 minutes, until the pork is just cooked through. Allow the pork to rest 10 minutes before slicing and serving.