Brush onions, peppers, and zucchini generously with some seasoned oil
Put garlic on a double layer of foil and drizzle with some seasoned oil, then wrap tightly in foil, twisting to seal
Put vegetables on a large tray
Brush sausages lightly with some seasoned oil and put on a plate
Prepare grill for direct-heat cooking over medium-hot charcoal (medium-high heat for gas)
Grill vegetables, garlic, and sausages, in batches if necessary, covered only if using a gas grill, turning occasionally (frequently for garlic), until tomatoes are charred and slightly wilted, 3 to 5 minutesutes; bell peppers are blackened in spots, 8 to 10 minutesutes; onions and zucchini are tender, 8 to 12 minutesutes; garlic is softened and caramelized in spots, about 10 minutesutes; sausages are browned and cooked thourough, 12 to 15 minutesutes
Transfer as cooked to a platter (or platters) and keep warm, covered
Peel peppers
Coarsely chop 2 bell pepper halves and 2 zucchini halves and transfer to a blender