Preheat oven to 425 degrees. Spray a large baking sheet with cooking spray, then set aside.
Clean potatoes under water. Dice them into ~1/2 inch pieces, then put them in a large bowl.
Add the onions, peppers, corn starch, garlic powder, salt, pepper and olive oil, then mix well, making sure that every potato piece has seasoning.
Pour into baking dish, spreading evenly.
Roast uncovered 25-35 minutes, or until potatoes are tender and golden brown