Preheat the oven to 450°F. Slice the squash, zucchini, peppers and carrots about ⅓ of an inch thick. Cut the onion in half and then into quarters. Line 2 baking sheets with aluminum foil and spray liberally with cooking spray.
Arrange the vegetables in a single layer on the baking sheets. [It's important that the vegetables are in a single layer, otherwise, they'll steam and won't become golden brown.]
Place the squash, zucchini, and carrots together, they will take the longest to bake.
On a separate pan, place the quartered onions and pepper slices. Spray the vegetables with cooking spray or drizzle lightly with olive oil.
Season with garlic salt, Italian seasoning and freshly ground black pepper to your taste.
Place the carrots, squash and zucchini into the oven first and bake for 10 minutes.
Add the onions and peppers and continue to cook at 450°F for another 10 minutes. After a total of 20 minutes, increase the temperature to 475°F. Continue to cook another 10 minutes or until the vegetables are golden brown.
Remove from the oven and let them cool on the pan for 2 minutes.
Toss together gently and garnish with the chopped onions before serving.