Mix your garlic salt and black pepper in a small container and mix. Pour your olive oil over the top of the potatoes. Use a wooden spoon or I even use the same measuring cup from the oil to mix them around and get the oil covered over all of them. Sprinkle a bit of the garlic salt and pepper mixture and a bit of the rosemary over the top, mix, and continue to do so until the entire batch of potatoes is covered with the spices and oil.