In a small saucepan heat vinegar, sugar, and 1/4 teaspoon of the salt just to boiling, stirring to dissolve sugar. Remove from heat. Stir in red pepper and onion. Let stand 30 minutes, stirring occasionally.
Meanwhile, preheat oven to 450 degrees F. Coat a baking pan or roasting pan with cooking spray. Remove stems from mushrooms and use a spoon to scrape out gills. Wipe mushrooms clean with a damp paper towel. Place mushrooms, stem sides up, in the prepared pan. Brush mushrooms with olive oil and sprinkle with the remaining 1/4 teaspoon salt. Roast 15 to 18 minutes or until tender. Blot excess moisture from mushrooms with a paper towel.
In a bowl stir together the next four ingredients (through black pepper0. Stir 1 1/2 teaspoons of the liquid from the onion mixture into mayonnaise mixture.
Coat an extra-large nonstick skillet with cooking spray. Heat over medium heat. Break eggs into skillet. Reduce heat to low to medium-low; cook 3 to 4 minutes or until whites are completely set and yolks start to thicken. Turn eggs and cook to desired doneness (30 seconds for over easy or 1 minute for over hard).
Drain onion mixture, discarding marinade. Spread mayonnaise mixture on toasted bread slices. Top with spinach and mushrooms. Add a fried egg and spoon onion mixture over each.