Typically the best "french fries" (American term) are obtained if you cut the potatoes and boil them in water for just a few minutes, then deep fry them in oil.
Pre heat oven to 200c
Cut your potatoes longways into eights. Leave the skin on.
Par boil them for five minutes
Spread out on a baking tray and toss in olive oil and thyme.
Bake for 35 - 40 minutes (turning halfway), or until nice and golden.