Mix 2 teaspoons each onion powder and garlic powder, 1 teaspoon each dried dill and kosher salt and 1/2 teaspoon pepper. Cut 3 sweet potatoes into 3/4-inch-thick wedges. Toss with 2 tablespoons vegetable oil and 1 1/2 tablespoons of the spice mixture. Roast at 450 degrees F until browned, about 50 minutes. Toss with the remaining spices. Serve with ranch dressing.