Preheat the griller to high. Cut the capsicums lengthways into eight, then remove the stems and any seeds. Trim the zucchini and slice diagonally. Trim the fennel, then cut lengthways into thin slices. Peel the onion and slice into rings.
Cover the griller rack with a single layer of the vegetables, positioning the capsicums skin side down. Brush with some of the olive oil and grill, on one side only, until lightly browned but still slightly firm. If necessary, cook the vegetables in batches and keep the first batch warm in the oven.
Meanwhile, peel and halve the cloves of garlic and halve the tomato. Cut four thick slices of bread and toast on both sides.
Rub the top of each bread slice with the cut garlic and tomato, then pile the grilled vegetables on top. Trickle over the remaining olive oil and season the bruschetta with salt and pepper to taste. Tear the basil leaves and scatter over the top; serve while still warm.