1. Cut yellow squash and zucchini cut them into a total of evenly cut thirty-two 3/4-inch pieces.
2. In a large saucepan of boiling salted water cook carrots 1 minute.
3. Add yellow squash and zucchini and cook vegetables 5 minutes. Transfer vegetables with a slotted spoon to a large bowl of ice and cold water to stop cooking and drain well in a colander. Transfer vegetables to a bowl.
4. In boiling water remaining in pan cook onions 4 minutes and transfer with slotted spoon to bowl of ice and cold water. Drain onions well in colander and peel, leaving root ends intact. Vegetables may be boiled 1 day ahead and chilled in sealable plastic bags.
5. In a small bowl whisk together vinegar, mustard, oil, and salt and pepper to taste. Basting sauce may be made 1 day ahead and chilled, covered.
6. Prepare grill.
7. Thread vegetables, alternating them, onto skewers.
8. Brush one side of kebabs with about half of sauce and grill, coated side down, on an oiled rack set 5 to 6 inches over glowing coals 5 minutes. Brush kebabs with remaining sauce and turn.
9. Grill kebabs 5 minutes more, or until squash is tender.

COOK’S NOTE: kebabs may also be grilled in a hot well-seasoned ridged grill pan over moderately high heat.