Preheat oven to 425 degree F. In a 13x9x2-inch baking pan, combine red onions, yellow summer squash, zucchini, sweet peppers, and garlic.
In a screw-top jar, combine parsley, balsamic vinegar, olive oil, oregano, salt, and black pepper; cover and shake well. Pour over the vegetables; toss gently to coat.
Roast about 25 minutes or until vegetables are crisp-tender, stirring twice. Makes 8 servings.