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Set the oven rack in the middle position and preheat the oven to 425°F. Prick the sweet potato with a fork and bake in a small baking dish until tender when pierced with a knife, about 1 hour. Keep warm.
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Meanwhile, pat the scallops dry and season with salt and pepper. In a large sauté pan over medium-high heat, warm the oil until shimmering, then add the scallops in one layer. Cook, undisturbed, for 2 minutes. Flip the scallops, add 2 tablespoons of the butter and the onion, then swirl the pan occasionally, letting the butter and onion get golden brown with a nutty aroma, about 2 minutes. Remove the scallops and keep warm. To the pan, add the vinegar and 3 tablespoons of the orange juice. Let it bubble and reduce to a semi-thick sauce, 1 to 2 minutes. Add the currants, spinach, and a generous pinch of salt and pepper and toss to coat. When the spinach starts to wilt, after about 1 minute, remove it the pan from the heat and keep warm.
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Working quickly, peel the sweet potato and discard the skin. In a bowl, roughly mash the sweet potato with the orange zest, a pinch of salt, the remaining 1 tablespoon butter, and the remaining 1 tablespoon orange juice.
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Divide the sweet potato mash between 2 plates, top each with half of the wilted spinach mixture and half of the scallops. Drizzle the pan juices over the scallops, season with salt and pepper, and serve.