Although you only use half of the bell pepper in this pizza, go ahead and roast both halves; use the other half in a sandwich.
Place bell pepper half, skin side up, on a foil-lined baking sheet; flatten with hand. Broil 15 minutes or until blackened. Place in a zip-top plastic bag; seal. Let stand 10 minutes. Peel and finely chop. Combine bell pepper, juice, and garlic, stirring well.
Preheat oven to 450°.
Heat a small nonstick skillet coated with cooking spray over medium-high heat. Add mushrooms and shallots; sauté 2 minutes or until tender. Remove from heat; stir in vinegar.
Place crust on a baking sheet coated with cooking spray. Spread bell pepper mixture evenly over crust, leaving a 1/2-inch border. Sprinkle basil evenly over bell pepper mixture; top with spinach. Spoon mushroom mixture evenly over spinach; sprinkle with cheese.
Bake at 450° for 4 minutes or until cheese melts. Sprinkle with black pepper and salt.