Baked Vegetable Chips with Yogurt Ranch Dip

Preheat the oven to 200 degrees F. Coat three baking sheets with cooking spray. Set aside. Slice the zucchini, parsnip, and sweet potato into 1/8 inch thick round slices. Use a mandolin for easy and consistent slicing.  Line one baking sheet with the sweet potato slices in a single layer. Line a second baking sheet with the zucchini in a single layer, and the third sheet with parsnips in a single layer. Spray the vegetables with cooking spray and season them with salt (optional). Bake for 1 hour and then rotate the baking sheets. Bake until the vegetables are crisp and dry, about 30 to 60 minutes more.  While the vegetables are baking, whisk together the yogurt, milk, and ranch powder. Refrigerate the ranch dip until needed.  Let the vegetable chips cool before serving. Once the chips have cooled completely, store them in an airtight container.