Heat a grill pan over medium-high heat. Remove gills from mushrooms using a spoon; discard gills. Combine 1 1/2 tablespoons oil and half of garlic; brush over mushrooms and onion slices. Place mushrooms and onion slices in pan, and cook for 5 minutes on each side or until tender. Remove from heat. Sprinkle mushrooms with 1/8 teaspoon salt. Separate onion slices into rings.
Place bread slices on a baking sheet, and broil for 1 minute on each side or until toasted.
Place remaining 1 1/2 teaspoons oil, remaining garlic, remaining 1/8 teaspoon salt, bell peppers, almonds, and vinegar in a food processor, and process until smooth. Spread red pepper sauce evenly over 1 side of each bread slice; top 4 bread slices evenly with cheese, 1 mushroom, onion rings, spinach, and basil. Top with remaining 4 bread slices, sauce side down.