Preheat the oven to 180°C.
Boil the raw/frozen dumplings in salted water and drain. If the potato dumplings have already been cooked, move to the next step.
Transfer the dumplings into a large casserole dish. Cut the red onion into large pieces, slice the sausage and put it into a dish with potato dumplings. Drizzle over with one tablespoon of oil, sprinkle with oregano and mix. Put in the oven and bake for 25 minutes.
Meanwhile, cut the zucchini into thin slices, wash the cherry tomatoes and drain the mozzarella balls.
After 25 minutes, remove the dish from the oven. Add the zucchini, cherry tomatoes and mozzarella evenly on top of the casserole. Bake for another 10-15 minutes, or until the mozzarella is melted. Garnish with a few basil leaves. Enjoy your meal!