Preheat the oven to 180°C (350°F). Grease a baking dish with olive oil.
Heat a pan over medium heat and sauté the onions and mushrooms until soft. Add the sliced sausages and cook for another 2 minutes.
Layer half of the sliced potatoes in the baking dish. Add the sausage and mushroom mixture, followed by the chopped tomato and spring onions.
Layer the remaining potatoes on top.
In a bowl, whisk together the eggs, sour cream, salt, and black pepper. Pour the mixture over the casserole.
Sprinkle grated Parmesan cheese on top.
Bake for 35-40 minutes until golden brown and the potatoes are tender.
Let it cool slightly before serving. Enjoy warm with a side salad or crusty bread.