Prepare the potatoes. Place the potatoes in a baking dish that will hold them snugly, in a single layer (more or less), and with enough height to cover them with the oil, without getting too near the top edge.Add the rosemary sprigs to the baking dish, placing some underneath the potatoes.
Add olive oil. Pour the olive oil over the potatoes evenly -- they should all be completely, or almost completely submerged in the oil.
Slowly bake. Place the potatoes in the preheated 225°F oven, uncovered, for 2 hours. Remove them from the oven and let them cool at room temperature until the oil stops sizzling.
Drain. Drain the potatoes over a bowl. (Save the rosemary infused oil -- you can use it for all sorts of cooking!)
Add the potatoes to a serving bowl, or return them to the baking dish and gently toss them with the salt to taste. (Here's How to Season to Taste.)