Pre-boil your potatoes. Cut your potatoes into 1.5 inch chunks and add them to a large pot. Add enough water to just cover the potatoes and bring to a boil, uncovered.
Preheat oven to 425 degrees and spray a baking sheet with olive oil cooking spray. In a large mixing bowl, toss the potatoes with the 1/2 tbsp olive oil, salt, pepper and rosemary.