Preheat the oven to 425 degrees Fahrenheit. Line a rimmed baking sheet with parchment paper.
Place the carrots on the prepared baking sheet. Drizzle the oil over the carrots and sprinkle on the thyme, cumin, salt, and pepper. Toss the carrots well so they are evenly coated. Spread them out into one even layer with space between each carrot. Roast for 15 minutes.
Meanwhile, whisk together the orange juice and maple syrup. After the carrots have roasted for 15 minutes, pour the orange maple mixture over the carrots, toss well, and spread back out evenly in one even layer. Roast for another 5-10 minutes until the carrots start to brown and caramelize and are tender when pierced with a fork or knife.
While the carrots are roasting for the final time, toast the walnuts. Heat a dry skillet over medium-high heat. Add the walnuts and toast, stirring frequently to prevent burning, until fragrant and starting to darken. This should take about 3-5 minutes. Watch them closely so they don't burn. Once toasted, pour them out onto a plate in one even layer to cool down. They will continue to crisp up as they sit.
To serve, place the carrots on a plate or platter, sprinkle over the toasted walnuts and extra thyme, if desired.