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1
Position a rack in the upper third of an oven and preheat to 400 degrees F.
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2
Prick the potatoes with a fork and place them directly on the oven rack. Bake until tender when pierced with a small knife, about 1 hour. Remove the potatoes from the oven and let cool for 5 minutes. Reduce the oven temperature to 350 degrees F.
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3
In a large, heavy pot over medium heat, melt 2 tablespoons of the butter. Add the garlic and shallot and saute until the shallot is translucent, about three minutes. Increase the heat to medium-high, add the spinach, and toss until wilted but still bright green, about four minutes. Transfer the spinach mixture to a sieve set over a bowl and press all the liquid out of the spinach.
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4
Using a serrated knife, cut a slice 1/2 inch thick off one long side of each potato and discard. Scoop out the potato flesh into a bowl, leaving a shell 1/4 inch thick. Add the sour cream, salt, pepper and the remaining 4 tablespoons butter to the potato flesh and mash with a potato masher to blend. Stir in 2/3 cup of the cheese and then the spinach. Spoon the potato mixture into the potato shells, mounding it high. Press the remaining cheese on top of the filling.
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5
Transfer the potatoes to a baking sheet and bake until heated through, about 20 minutes. To test for doneness, stick a small knife into a potato and leave it there for about 15 seconds. Remove the knife and feel the blade; if it is hot, the potatoes are ready. Serve immediately. Serves 4.
