Preheat the oven to 390F / 200C and line a baking sheet with parchment paper.
Whisk together the olive oil, dried thyme, dried sage and garlic powder.
Peel and dice the sweet potatoes into bite-size pieces of a similar size. Scrub the beets well, remove the tops and ends and cut into halves. Then dice the halves into 2-3 pieces, depending on their size. De-core the apple and cut into quarters, then halve the quarters.
Add the sweet potatoes, beets and apple to the baking sheet. Drizzle half the herb oil over the vegetables and toss to coat. Spread into a single layer on the baking sheet.
Pat dry the chicken thighs with paper towel, then nestle in between the fruits and vegetables. Pour the remaining oil over the chicken thighs and season with salt and black pepper, if using.
Bake the chicken and sweet potatoes sheet pan for 35 minutes, or until the chicken reaches an internal temperature of 165F using a meat thermometer and the sweet potatoes and beets have some browning and are fork tender.
Optional: for a fresh herb dressing, whisk together olive oil, apple cider vinegar, fresh finely diced rosemary and fresh thyme. Drizzle over the chicken and sweet potatoes when serving.