Combine oil, mustard basil and garlic in small bowl.
Cut eggplant and zucchini lengthwise into 1/4-inch thick slices. Cut onion and tomatoes crosswise into 1/2-inch-thick slices. Cut red pepper lengthwise into 2-inch wide pieces; discard seeds. Place vegetables on platter. Baste with reserved mustard mixture.
Place vegetables on oiled rack or vegetable basket. Grill over medium-high heat 3 to 5 minutes or until vegetables are tender, basting and turning once.
Layer vegetables on 4 slices of bread; cover with remaining 4 slices of bread.