Preparation Position 1 rack in center and 1 rack in bottom third of oven and preheat to 375°F. Finely chop tomatillos in processor. Add cilantro and blend until coarsely chopped. Add 1/2 cup water and next 12 ingredients; blend tomatillo sauce until smooth. Place 2 shallow ovenproof bowls on each of 2 large rimmed baking sheets. Divide half of mushrooms among bowls; sprinkle lightly with salt and pepper. Drizzle 11/2 teaspoons oil, then 2 tablespoons tomatillo sauce over each. Top each with 1 fish fillet; sprinkle with salt and pepper. Top with remaining mushrooms and tomatillo sauce. Drizzle 1/2 teaspoon oil over each. Cover baking sheets with foil, sealing sheets at edges. Bake until fish is just opaque in center, about 25 minutes.
Preparation Position 1 rack in center and 1 rack in bottom third of oven and preheat to 375°F. Finely chop tomatillos in processor. Add cilantro and blend until coarsely chopped. Add 1/2 cup water and next 12 ingredients; blend tomatillo sauce until smooth. Place 2 shallow ovenproof bowls on each of 2 large rimmed baking sheets. Divide half of mushrooms among bowls; sprinkle lightly with salt and pepper. Drizzle 11/2 teaspoons oil, then 2 tablespoons tomatillo sauce over each. Top each with 1 fish fillet; sprinkle with salt and pepper. Top with remaining mushrooms and tomatillo sauce. Drizzle 1/2 teaspoon oil over each. Cover baking sheets with foil, sealing sheets at edges. Bake until fish is just opaque in center, about 25 minutes.