Prepare the potatoes. Peel, then dice them into 1,5 cm pieces.
Heat the butter and oil in a pan over medium heat, taking care not to let the butter burn. Add the rosemary sprig and diced potatoes. Sauté about 5 min, with frequent stirring, until the potatoes are a light golden colour.
Add salt, remove the rosemary sprig, cover and cook over low heat about 15-20 min, or until the potatoes are soft.