4 large russet potatoes, scrubbed and patted dry
6 tablespoons (3/4 stick) unsalted butter
2 large garlic cloves, minced
1 large shallot, minced
2 bunches fresh spinach leaves, tough stems removed and leaves rinsed well
2 tablespoons sour cream
1/2 teaspoon salt
1/4 teaspoon freshly ground pepper
1 cup firmly packed shredded extra-sharp cheddar cheese
Position a rack in the upper third of an oven and preheat to 400 degrees F.
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.
In a large, heavy pot over medium heat, melt 2 tablespoons of the butter. Add the garlic and shallot and sauté 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.
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.
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.