The contrast of hot caramelized sweet potatoes with cold lemony arugula, radicchio and fennel makes this the ideal salad. It’s both warming and refreshing, and the savory shaved Parmesan on top ties it all together.
Heat the oven to 350 degrees. Line a rimmed baking sheet with foil.
Toss the sweet potato halves with 3 tablespoons olive oil and a generous pinch each of salt and pepper on the prepared sheet. Arrange in a single layer, cut sides down, spacing about an inch apart. Scatter the thyme evenly over the sweet potatoes. Roast until cut sides are browned and a paring knife slides easily through a sweet potato, 45 to 55 minutes.
Meanwhile, combine one quarter of the sliced dates with ¼ cup water in a small saucepan. Bring to a simmer over medium heat, then cover and reduce the heat to low. Cook, stirring occasionally, until the dates have absorbed all the water and are soft enough to smash, about 15 minutes. (If the pan dries out before the dates are soft, add another tablespoon or two of water.) Use a fork to smash the dates into a coarse paste. Add the butter to the saucepan and smash until evenly mixed. It’s OK if the butter melts in the process.
Place the radicchio in a large bowl and cover with ice-cold water. Let soak for 10 minutes to temper the bitterness. Add the fennel to the bowl, submerging it in the water and adding more water if needed to cover. Let soak together for 5 minutes to crisp the fennel. After soaking, drain both the radicchio and fennel in a colander, shaking well to dry. Refrigerate both in the colander until ready to serve.
Remove the sweet potatoes from the oven and carefully flip all the sweet potatoes cut sides up. Raise the oven temperature to 425 degrees. Divide the date butter among the cut sides and gently spread evenly. Sprinkle with salt and pepper. Return to the oven and roast until the date butter caramelizes on top, 8 to 10 minutes.
Transfer the radicchio and fennel to a large bowl and add the arugula, Mandarin oranges and remaining sliced dates. Drizzle with the remaining 2 tablespoons olive oil and sprinkle with salt and pepper. Toss to evenly coat. Drizzle the lemon juice over the salad and toss to coat again.
Divide the sweet potatoes among serving plates and top with the salad and Parmesan shavings. Serve immediately.
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