1. Heat grill or grill pan to medium-high. Spread eggplant slices on a sheet of paper towels and sprinkle with 1/4 tsp of the salt. Set aside to drain. Blot with paper towels.
2. In a small bowl, blend remaining 1/4 tsp salt, the vinegar and mustard. While whisking, add oil in a thin stream. Whisk in pepper.
3. Brush eggplant and onion with some of the dressing and grill 4 minutes per side, turning once. Remove to a platter. Brush zucchini with dressing and grill 3 minutes per side, turning once (this may need to be done in batches). Transfer to platter. Refrigerate overnight.
4. Heat oven to 350 degrees. Slice bread lengthwise in half. Spread each cut side with 1/4 cup of the pesto. Divide eggplant slices between loaves. Top with zucchini, onion, cheese and roasted peppers. Replace top crusts and wrap loaves in foil.
5. Bake at 350 degrees for 20 minutes, weighing down with a cast-iron skillet or foil-wrapped brick. Unwrap loaves and bake an additional 10 to 15 minutes, until cheese is melted and sandwiches are warm throughout. Cut each loaf into 8 slices.