Preheat your oven to 400°F (200°C).
In a small bowl, mix the juice of 1/2 lemon, olive oil, chopped rosemary, thyme, garlic powder, salt, and pepper.
Place the chicken breast in a shallow dish and coat evenly with the lemon-herb mixture. Let marinate for at least 15 minutes.
Meanwhile, peel and dice the sweet potato into 1/2-inch cubes and tear the kale into bite-sized pieces, removing tough stems.
On a baking sheet, toss the sweet potato cubes and kale with a drizzle of olive oil, salt, and pepper. Spread them out in an even layer.
Place the marinated chicken breast on another baking sheet or in a roasting pan.
Roast both the chicken and the vegetables in the oven. Bake the chicken for 20-25 minutes, or until it reaches an internal temperature of 165°F, turning once if needed. Roast the sweet potato and kale for 20-25 minutes until the sweet potatoes are tender and the kale becomes crispy.
Remove from the oven and let the chicken rest for a few minutes before slicing. Serve the sliced chicken with a side of crispy roasted kale and sweet potatoes.