Heat a grill pan over medium-high heat. Remove the gills from the mushrooms using a spoon; discard gills. Combine 1 1/2 tablespoon oil and half of the garlic; brush over the mushrooms and onion slices. Place the mushrooms and onion slices in the pan and cook for 5 minutes on each side or until tender. Remove from the heat. Sprinkle mushrooms with 1/8 teaspoon salt. Separate the onion into rings.
Place the bread slices on a baking sheet, and broil for 1 minute on each side or until browned.
Place the 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 the red pepper sauce 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, spread side down. Enjoy!