To make the dressing, combine the lemon juice, oil, salt, and pepper in a small bowl, beating with a whisk until blended. Brush the cut side of the top half of the bread with 1 tbsp of the dressing and set aside.
Layer the remaining bread with the lettuce, bell pepper, and radishes; drizzle with 2 teaspoons of the dressing. Layer with the onion and zucchini and drizzle another 2 tsp of dressing. Layer with the tomatoes and cheese and drizzle with the remaining dressing. Add the basil and cover with the reserved bread, cut side down, pressing it firmly.
With a serrated knife, cut the loaf crosswise into 6 pieces.