Shiitakes have tough inedible stems that should be removed. For white mushrooms (called button when small), cremini, and portobellos, trim the spongy tip; the rest is edible.
Flour, for rolling out puff pastry
1 (from a 17.3-ounce package) sheet frozen puff pastry, thawed according to package instructions
1 medium onion, halved and thinly sliced
2 tablespoons olive oil
Coarse salt and ground pepper
2 packages (10 ounces each) white mushrooms, trimmed and thinly sliced
1 package (10 ounces) fresh baby spinach
2 ounces soft goat cheese, crumbled
1. Preheat oven to 400 degrees. On a floured surface, roll the puff pastry out to a 16-by-10-inch rectangle. Trim uneven edges. Place the pastry on a baking sheet. With a sharp knife, lightly score dough to form a 1-inch border. Using a fork, prick dough inside the border every 1/2 inch. Bake until golden, rotating pan once, about 15 minutes or until tender.
2. Meanwhile, in a small saucepan with a tight-fitting lid, toss onion with 1 tablespoon oil. Season with salt. Cover and cook over medium heat until onion begins to brown, about 5 minutes. Stir. Continue cooking with cover on for 15 minutes, stirring every 5 minutes. Set aside.
3. In a large saucepan with a tight-fitting lid, heat remaining oil. Add mushrooms; cover and cook until tender and all liquid has evaporated, about 10 minutes. Fold in spinach; season with salt and pepper. Cover and cook until wilted, about 5 minutes more. Drain any liquid
4. Top dough with mushroom-spinach mixture. Scatter onion and goat cheese on top. Bake until cheese is lightly browned, about 15 minutes.
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