For each serving, to order: Season 1 fish fillet with salt and pepper.
In saute pan over medium-high heat, sauté 3 ounces assorted mushrooms in 1 tablespoon butter until soft. Add 2 tablespoons shallots and 1/2 teaspoon garlic; cook about 1 minute. Add 1 ounce oyster mushrooms, 1/3 cup stock, 1 tablespoon wine and 1 teaspoon each soy sauce and lemon juice. Bring to boil; reduce by half. Add 1 tablespoon butter. Shake pan until butter melts and sauce thickens slightly. Season with salt and pepper; keep warm.
In saute pan, heat 1 tablespoon olive oil over medium heat. Add fish fillet; sauté 3 to 4 minutes on each side until skin is crisp and fish is cooked through; remove and keep warm.
In saute pan, heat 1/2 tablespoon butter. Add 4 ounces spinach; toss until wilted. Plate spinach; place fish on top. Spoon mushroom sauce around fish.