Preheat oven to 400 degrees. Take chicken out of the fridge to come up to room temperature.
Place the sweet potatoes, onion and sage in a roasting pan. Drizzle with 1 tbsp olive oil and sprinkle with salt.
Place chicken on top of potatoes. Sprinkle chicken with salt and pepper as well.
In a small bowl, mix together remaining 2 tbsp olive oil, mustard and maple syrup. Pour this on top of the chicken, coating it well.
Place in the oven and roast for 60 minutes or until temperature of chicken reaches 165 and the potatoes are soft. This is also very good re-heated!