Toss potatoes in oil and roast in a baking dish or on a sheet pan for 30 minutes.
Remove from oven, add salt and rosemary, and stir around well.
Roast for another 10 minutes, up to another 30, depending on how crispy you want 'em!
Notes
Do not peel - these are too small to fuss with, plus that's where your nutrients are!
If using tallow and it doesn't want to mix in at first, I just pop the whole tray in the oven for 5 minutes until it is melted and then stir it around and finish the first 30.