1. Preheat grill to medium-high heat (350° to 400°) or heat grill pan over medium-high heat.Combine 1 tablespoon lemon juice, 2 tablespoons olive oil, and 1/4 teaspoon each kosher salt and black pepper in a bowl. Add chicken, tossing to coat. Let marinate 10 minutes.
2. Grill bread, turning once, 3 minutes or until toasted; set aside. Brush romaine halves evenly with 1 tablespoon olive oil. Grill romaine halves 1 to 2 minutes or until grill marks appear. Set aside. Grill peppers, turning once, 5 to 7 minutes or until tender. Cut peppers into strips, and set aside.
3. Grill chicken, turning once, 8 minutes or until cooked through.
4. Divide grilled peppers and chicken among bread slices; top with mozzarella. Drizzle romaine halves with remaining 1 tablespoon olive oil and remaining 1 tablespoon lemon juice. Sprinkle with pine nuts, kosher salt, and black pepper.