Soak wooden skewers for at leat 30 minutes in water.
Whisk dressing ingredients (oil- mustard) in a jar and set aside.
Alternate vegetables (eggplant, zucchini and summer squash) with tomatoes on skewers. Each skewer should hold approximately 6 vegetables (2 of each eggplant, zucchini and summer squash rounds) and 6 tomatoes.
Arrange skewers on baking sheet while you complete all 10 skewers. Brush skewers with dressing. (If making ahead refrigerate for up to 8 hours- refrigerating dressing and skewers separately) Prepare barbecue (or preheat broiler).
Brush skewers with dressing, sprinkle with salt and pepper (if desired).
Grill (or broil) until skewers are tender and charred slightly, turning often (about 10 minutes).
Transfer to serving platter and brush again with dressing.