Peel both types of potatoes and cut into small cubes - like sugar cubes. First pour the oil into the baking dish, then put the potato cubes in the dish. Mix the potato cubes with a spoon and distribute the oil over the potatoes.
Now salt and pepper the oiled potato cubes to taste and then pour the rosemary over them. Place the casserole dish on the middle rail for approx. 20 - 25 minutes in the oven preheated to 200 ° C top / bottom heat. Check the potatoes to avoid serving half raw potatoes.
When the potatoes are no longer raw, spread the previously diced feta on top and put the pan in the oven again for 6 - 7 minutes.