To prepare dressing, combine first 8 ingredients in a small bowl, and stir well with a whisk.
To prepare salad, heat 1 teaspoon oil in a large nonstick skillet over medium-high heat. Add mushrooms and 1 cup onions; sauté 8 minutes. Stir in corn; remove from heat.
Place the fish on a foil-lined baking sheet; brush evenly with 1 teaspoon oil. Broil 8 minutes or until fish flakes easily when tested with a fork.
Place 2 cups spinach on each of 4 plates; top each serving with 1/4 cup bean sprouts, 1/4 cup red bell pepper, 1/2 cup mushroom mixture, and 1 fillet. Drizzle about 2 tablespoons dressing over each salad.