1. Fill a medium pot with water and bring to a boil. Add squash and cook until just tender. Drain squash and place in a large bowl along with peppers, zucchini, garlic, and onion in a large bowl. Add oil, verjus, and rosemary, and season with salt and pepper; toss to combine.

2. Preheat a grill pan over high heat. Drain vegetables and place on grill pan, working in batches if necessary. Cook vegetables until tender.