Preheat oven to 400F. Peel and cut potatoes into cubes. Thinly slice onions. Combine both ingredients in a baking pan. Drizzle with olive oil, salt and pepper and toss to combine.
Bake for 40 minutes. Stir the potatoes halfway through.
Meanwhile, rinse cherry tomatoes and cut in half. Cut mozzarella into cubes. Alternately, you can shred it with a cheese grater.
Remove the baking pan from the oven and top the potatoes evenly with the halved cherry tomatoes and mozzarella. Sprinkle with oregano.
Return to the oven and bake for 10 minutes longer until the mozzarella has melted and the tomatoes are blistered. Serve hot or warm.
You can serve this potato dish as an appetizer, side dish or main dish. It is delicious whether it is hot or warm. If you're not fond of oregano, try substituting one of your favorite herbs such as rosemary or thyme. This dish would make a great brunch dish served with a fried egg on top.