Blend first 4 ingredients in processor until smooth. Season with salt and pepper. Spray grill with oil spray; prepare barbecue (medium heat). Spray mushrooms and onion slices with oil spray; sprinkle with salt and pepper. Grill mushrooms, onion and bread until vegetables are tender and bread is golden, turning often, about 12 minutes for vegetables and 5 minutes for bread. Transfer 2 tablespoons red pepper mayonnaise to large bowl. Whisk in vinegar, add spinach and toss. Divide among 4 plates. Place 1 bread slice atop spinach on each. Top each with 1 mushroom, then 1 onion slice. Spoon 2 tablespoons red pepper mayonnaise atop each.