Preheat oven to 400 F. Move the rack to the middle position.: You will feel the warm pulse of heat as the oven reaches temperature, and placing the rack in the middle position ensures even air circulation so nothing browns too quickly on one side. This temperature is ideal for turning the surface of the sweet potatoes golden while allowing the centers to become tender. A common mistake is crowding the pan, which prevents crisping, so give the pieces space on the sheet. If your oven has hotspots, rotate the pan once during the roast for uniform color.
Prep the sweet potatoes. Add them to a large baking sheet and toss with the oil, garlic powder, chili powder, cumin, onion powder, paprika, and salt & pepper. Roast for 30 minutes, turning halfway.: As the sweet potatoes roast you will notice a warm, toffee like aroma and small steaming sounds as moisture escapes. The oil and spices help create a glossy coating that crisps at the edges, while the interior steams to a tender texture. When you turn them halfway, look for deepening color and slightly caramelized spots; those visual cues indicate the sugars are developing. Avoid using too small a pan, which leads to steaming instead of roasting, and check for fork tender centers to ensure they are cooked through.
Meanwhile, prep your other ingredients. If you want to warm the tortillas, wrap the stack of them in tin foil and pop them in the oven during the last 10-15 minutes that the sweet potatoes are cooking.: While the potatoes soften, preparing toppings fills the kitchen with fresh, contrasting scents from the cilantro and lime . Warming the flour tortillas in foil lets steam soften them, making them flexible and pillowy. You should hear a faint sizzle as the tortillas warm, and they will smell toasty and inviting. A typical error is overheating them directly on a flame which can char and dry them, so the oven method keeps them uniform and warm without burning.
Load the tacos up and enjoy!: At assembly, you will enjoy the contrast between hot roasted sweet potatoes and cool, creamy Greek yogurt and avocado , with pops of acidity from the tomatoes and lime . Build each taco with a balance of textures so each bite contains soft, creamy, and crisp elements. If the tacos feel too dry, add more Greek yogurt or a squeeze of lime for moisture and brightness. A frequent slip is overfilling tortillas, which makes them hard to eat, so distribute the filling evenly and taste one before serving the rest.
