Preheat oven to 425 degrees F.
Scrub the potatoes, pat them dry. Prick the potatoes a few times with a fork and bake them in the middle of oven for 45 minutes to 1 hour.
While the potatoes are baking, in a heavy saucepan cook the spinach in the water clinging to the leaves, covered, over moderately high heat, stirring occasionally, for 3 minutes, or until it is wilted. Drain the spinach in a colander set over a bowl, reserving the cooking liquid, and chop it coarse.
In a bowl, combine the spinach and the butter. While the potatoes are warm, halve them lengthwise, scoop them out, leaving the shells as thin as possible, and chop the scooped-out potato coarsely. Add the chopped potato to the bowl with the spinach, combining the mixture well. Add the lemon juice and stir in 1 cup of the cheese and 1 tablespoon of the reserved cooking liquid, or enough to reach a smooth consistency, Season the mixture with salt and pepper, divide it among the potato shells, and top it with the remaining 1/4 cup cheese.
Bake the potatoes in a baking dish in the middle of a preheated 425 degree oven for 10 minutes, or until they are heated through.
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