For Mardi Gras, party gras or just any day gras, this super-savory vegetarian sandwich is a real crowd pleaser. You can prepare it up to 12 hours in advance, and the flavors will only improve. To make it vegan, simply skip the cheese and use vegan pesto and tapenade.
Preheat the oven to 450° F with two racks close to the middle.
Slice the eggplants 1/4-inch thick. Spray a baking sheet generously with cooking spray and arrange the eggplant slices in a single layer. Spray tops of slices with cooking spray, sprinkle generously with salt, and place in preheated oven. Bake for 30 minutes, flipping the slices at the halfway point.
While the eggplant begins to bake, prepare the portobellos. Remove stems and, using a small spoon, scrape the gills off the underside of the mushroom caps. Slice caps 1/4-inch thick. In a small bowl, whisk together olive oil, lemon juice, smoked paprika, and 1/2 teaspoon salt. On a second baking sheet, toss the portobello slices with the dressing. Arrange in a single layer and add to oven. Bake for 20 minutes.
Cut across the beltline of the boule and remove the top half. Use your hands to tear most of the soft insides out of both halves of the bread, leaving about 1/2 inch barrier before the crust. (Tear the soft parts into small chunks and freeze for use later as fresh breadcrumbs -- just give them a whirl in the food processor when you need them.) Spread the olive tapenade inside the bottom half of the bread and the pesto inside the top half.
Arrange the portobello pieces in a compact layer on the bottom of the bread. Layer on the roasted peppers, eggplant, and cheese. Replace the top half of the bread and press down firmly a few times to make it nice and compact. Wrap and store for up to 12 hours if desired, then slice into wedges and serve.