Optional if using rosemary: While the potatoes roast, add the finely chopped rosemary to a small bowl and add just enough olive oil to make a thick, spoonable paste, ~ 1 scant tablespoon. After the 40 minutes, flip the potatoes again.
If not using rosemary: return the potatoes to the oven and roast for 7 minutes, or until potatoes are deeply golden brown and crisp.
If using rosemary: Scootch the potatoes close to one another and spoon the rosemary paste on top of them. Use a small spatula or spoon to spread out the rosemary as best you can. Spread out again. Roast for 7 minutes, or until potatoes are deeply golden brown and crispy.