1. Puree beans with rosemary, garlic,, and 4 tablespoons oil until smooth. Transfer mixture to a bowl. Salt and pepper to taste.
2. Remove sides from pepper, discarding seeds and stem. Put all the vegetables on a sheet pan and drizzle with remaining 1 tablespoon oil and salt and pepper to taste.
3. Grill mushroom caps, in a lightly oiled grill pan over moderately high heat, turning once until just tender, about 7 minutes. Transfer mushrooms to a plate.
4. Grill remaining prepared vegetables, turning, until tender, about 6 minutes total, and transfer to plate to cool slightly. Cut pepper, zucchini and onion into bite-size pieces.
5. Fill mushroom caps evenly with white bean mixture and top with remaining grilled vegetables. Serve warm or at room temperature.