1. Cut bread in half horizontally. Using a short serrated knife, carve out insides of bread, in whole pieces if possible. Using a long serrated knife, thinly slice inside pieces. Cover; set aside. Pull out any remaining bread from crusts, leaving 1/4-inch bread border. Cover crusts; set aside.

2. Thinly slice mushrooms. Heat a grill to medium hot, or place a grill pan over high heat. Brush mushrooms with olive oil; sprinkle with salt and pepper. Grill until tender, 2 to 3 minutes per side.

3. Brush fennel with oil; sprinkle with salt and pepper. Grill until tender, 3 to 4 minutes per side.

4. Arrange filling in bottom of bread: First, cover with half the tomatoes, then half the onions. Sprinkle with salt and pepper; drizzle with 3 tablespoons dressing. Cover with 1 cup basil, and top with a layer of sliced bread. The second layer is made up of half the mushrooms, then salt and pepper, dressing, basil, and bread. The third layer is peppers, salt and pepper, dressing, basil, and bread. The fourth layer is the remaining mushrooms, salt and pepper, dressing, basil, and bread. The fifth layer is fennel, salt and pepper, dressing, basil, and bread. The last layer is the remaining tomatoes, onions, salt and pepper, dressing, and basil.

5. Place top of bread on bottom half, enclosing vegetables. Wrap with plastic; weight with heavy books. Let stand 45 minutes.

6. Unwrap sandwich. Using a large serrated knife, cut into eight wedges.