Preheat the oven to 425 degrees.
Place the sweet potatoes on sheet tray and place in the oven. Bake until very soft and caramelized, about 1 hour and 15 minutes. Remove from the oven and let cool. Keep the oven on. Carefully peel the skin off each potato by hand, leaving each potato whole. Cut the potatoes into a large dice. Set aside.
In a large bowl, break up the dried biscuits into small pieces. Set aside.
Heat a medium-sized saucepan over medium heat. When hot, add the oil. Next, add the onion and sauté for 4 minutes or until the onion is transparent. Add the chicken stock, thyme, and pepper and bring to a boil. Pour the mixture over the broken-up bread. Stir to combine. Carefully fold in the sweet potatoes and mix to combine. Season with salt, to taste.
Grease a 2-quart casserole with butter. Transfer the dressing to the casserole and smooth the top over. Place in the oven and bake for 45 minutes or until firm and light brown. Check occasionally to make sure it is not browning too much. If the dressing begins to get too brown, cover the top with foil. Serve hot.