1. Preheat the oven to 375 degree F.
2. Place a large ovenproof pan on medium low flame and heat oil and butter in it.
3. Add onion, and cook for about 15 minutes or until soft.
4. Drop potatoes, shallots, and half the rosemary sprig.
5. Season with salt and pepper.
6. Turn the heat to medium high, and cook for about 10 minutes or until potatoes begin to brown.
7. Remove pan from heat. Throw remaining rosemary sprig and chicken stock in it.
8. Cover the pan with lid.
9. Pop it in oven, and cook for 10 minutes or until potatoes are nice and tender.
10. Remove the lid, and cook for additional 20 minutes or until liquid reduces.
11. Garnish potatoes with rosemary leaves and serve hot.
The cooking time depends on the size of potatoes.