Preheat the oven to 425ºF.
In a bowl, toss the potatoes with 1 tablespoon of the olive oil and season with salt and pepper.
Place the potatoes, in a single layer, on a baking sheet and place in the pre-heated oven.
Roast until fork tender, about 12-15 minutes.
While the potatoes are cooking, heat the rest of the olive oil and butter in a sauté pan.
Add the leeks and caramelize them. Add the rosemary and cook for 1 minute.
Toss with the roasted potatoes and serve.
