Step 1: Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper or aluminum foil.
Step 2: In a large mixing bowl, combine the cubed sweet potatoes, olive oil, maple syrup, cinnamon, garlic powder, salt, and black pepper. Toss everything together thoroughly until the sweet potato cubes are evenly coated.
Step 3: Spread the seasoned sweet potatoes in a single layer on the prepared baking sheet, ensuring they are not overcrowded. Roast in the preheated oven for 20 minutes. Halfway through, stir or flip the potatoes to ensure even roasting and caramelization.
Step 4: After the initial 20 minutes, sprinkle the chopped walnuts evenly over the sweet potatoes on the baking sheet. Return to the oven and roast for an additional 5 to 7 minutes, or until the sweet potatoes are tender and the walnuts are fragrant and lightly toasted. Watch the walnuts carefully to prevent burning.
Step 5: Remove the baking sheet from the oven and let the sweet potatoes cool slightly before serving. Garnish with fresh thyme or rosemary, if desired, and serve warm.