Preheat the oven to 400°F (200°C). Halve the sweet potato(es) lengthwise and place cut-side up on a baking sheet. Drizzle with half of the oil and season with salt and pepper. Bake for 40-50 minutes, until tender when pierced with a fork. [If short on time, you can microwave the sweet potatoes: Pierce the sweet potato(es) several times with a fork. Microwave on high for 6-10 minutes (depending on size), flipping halfway through, until tender. Slice in half and season with salt and pepper.]

Meanwhile, heat the remaining oil in a skillet over medium heat. Add the onion and cook until softened, 5-7 minutes. Stir in the spices and a pinch of salt and cook 1 minute longer, until fragrant. For a milder dish, reduce or omit the crushed red pepper. Add the ground beef, break it apart with a spoon, and cook until browned and fully cooked, 8-10 minutes.

Serve the sweet potato(es) topped with the spiced beef mixture and a dollop of Greek yogurt.