Preheat oven to 425°F and line the baking tray with aluminum foil to make clean-up easier.
Peel and dice the sweet potato into small cubes.
Spay the foil covered baking tray with oil and spread the sweet potato evenly over the tray.
Drizzle with 1/2 tablespoon of olive oil. Roast for 20 minutes or until tender. Remove from the oven and set aside.
Meanwhile, slice the brussels sprouts into 1/4 inch rounds and dice onion and celery into small pieces. Remove the thyme from its stem and chop.
Heat 1 tablespoon olive oil in a sauté pan over high heat and add brussels sprouts. Stir occasionally until caramelized.
Reduce the heat to medium low heat and add the onion. Cook slowly for 8 minutes and add the celery and continue cooking for another 6 minutes until onion is caramelized. As these ingredients cook, grate the apple.
Increase heat to high and add sweet potatoes, grated apple and thyme. Season with salt and pepper and continue cooking for 3 to 4 minutes until heated thoroughly.