1. Preheat oven to 450 F. On a rimmed baking sheet, toss mushrooms with 1 tablespoons oil and thyme leaves; season with salt and pepper. Roast until mushrooms are lightly browned. 15 minutes.
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2. Rub fish with 1 tablespoon oil; season with salt and pepper. Arrange fish over mushrooms; top with thyme sprigs. Roast until fish is opaque throughout; 10 to 12 minutes.
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3. Top fish with fresh lemon and parsley plus lemon wedges for serving.