Peel the potatoes and cut into large pieces. Put the finely chopped onions, oregano, salt, pepper and the olive oil in a casserole dish in the preheated oven at 180 degrees circulating air, 200 degrees top / bottom heat.
Just before the potatoes are done, do a toothpick test, mix with the torn mozzarella and tomatoes and top with the flaky butter. Put back in the oven until the cheese is chewy.
You can do without the cherry tomatoes. I know several variants of the recipe, sometimes with, sometimes without or with significantly fewer tomatoes.