Cut away most of the fat from the meat.
Cut two garlic cloves in small pieces.
Cut small holes in the meat and put the garlic pieces in the holes in the meat.
Place the meat in a bag and pour the marinade in the bag. Close the bag and shake it a bit.
Place the bag in the fridge for minimum one hour.
Peel and cut the potatoes in preferred pieces.
Boil the potatoes for ten minutes. Pour away the water from the casserole.
Divide the potatoes in four equal piles and place them on aluminum paper.
Divide the rosemary between the four potatoes on the aluminum.
Pour half a teaspoon of black pepper, one teaspoon of salt and one tablespoon of butter on each pile of potatoes.
Close the aluminum like a little bag.
Pour the sour cream, olive oil, one teaspoon of salt and half a teaspoon in a bowl.
Press one garlic clove with a garlic press in the bowl and stir well.
Pour away the marinade from the bag.
Place the aluminum with the potatoes and the meat on the grill for approx one hour.
Cut the meat in thin slices before serving.