1. Heat oven to 400° F. On a rimmed baking sheet, toss the mushrooms, potatoes, and sausage with the oil and ½ teaspoon each salt and pepper. Roast, tossing once, until the potatoes are tender, 20 to 25 minutes. Let cool, transfer to a bowl, and fold in the spinach and Gruyère.
2. Divide the dough into 6 pieces and, on a lightly floured surface, pull and roll each into a 6-inch round. Dividing evenly, spoon the spinach mixture onto one side of each round (about ⅔ cup per round), leaving a ½-inch border. Dot the border with water, fold the dough over to form a semicircle, and press firmly to seal; crimp, if desired.
3. Place the pockets on a parchment-lined baking sheet, cut several slits in each, and bake until golden brown, 20 to 25 minutes.
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