Stir in oil. Season with salt, onion powder, garlic powder, and pepper. Toss until evenly coated.  

Spoon vegetables into a 9- by 13-inch baking pan. Bake vegetables, uncovered, for 20 minutes. Mix, return to oven, and continue baking, stirring once, until vegetables are tender, about 40 minutes longer.

Preheat oven to 375°F. 

Peel and cube all vegetables.