Heat the oven to 400°. On a baking sheet, toss the sweet potatoes with 2 tablespoons of oil, the cumin, thyme, and garlic. Season with salt and pepper and roast, stirring occasionally. Meanwhile, in a small bowl, combine the lime juice and ginger and let stand for 10 minutes to soften. Whisk in the remaining 1⁄4 cup oil until emulsified and then with salt and pepper. To serve, place the collard greens in a large bowl and toss with 1 tablespoon of the dressing, massaging it into the greens. Transfer the greens to a serving platter, top with the sweet potatoes, and sprinkle with the goat cheese and cashews. Serve with the remaining dressing on the side.