Peel and slice the potatoes into thin rounds. Boil them in salted water for about 10 minutes until just tender, then drain and set aside.

Grate or thinly slice the zucchinis and sprinkle lightly with salt. Let them sit for 10 minutes, then pat dry with a kitchen towel to remove excess moisture.

Heat olive oil and a little butter in a skillet. Add the diced onion and cook until golden and caramelized. Add the minced garlic and cook for 1 minute more.

In a bowl, whisk together the eggs, sour cream, salt, and pepper. Stir in the mozzarella cheese, crumbled feta, and sliced spring onions.

Grease a baking dish with olive oil or butter. Layer half of the potatoes on the bottom, followed by zucchini, caramelized onions, and a sprinkle of cheese mixture. Repeat the layers, finishing with more cheese on top.

Pour the egg and sour cream mixture evenly over the layers, making sure everything is coated.

Bake in a preheated oven at 375°F (190°C) for 35–40 minutes, or until golden brown and bubbling on top.

Let it rest for 5 minutes before serving. Slice, serve warm, and enjoy!