Preheat your oven to 180°C (356°F) and grease a medium-sized baking dish with butter or olive oil.

Peel and thinly slice the potatoes, ensuring even thickness for uniform cooking. Boil the slices in salted water for 5–6 minutes until slightly tender but not fully cooked. Drain and set aside.

Heat olive oil in a skillet over medium heat. Add the chopped onion and sauté until translucent. Stir in the minced garlic and cook for another 30 seconds until fragrant.

Add the chopped spinach to the pan and cook until wilted. Season with salt, pepper, and nutmeg. Remove from heat and let cool slightly.

In a small bowl, mix the cooking cream with thyme and a pinch of salt and pepper.

Begin layering the casserole: spread half of the potato slices at the bottom of the baking dish, followed by the spinach mixture, then sprinkle with mozzarella and Parmesan cheese. Add another layer of potatoes on top and pour the cream mixture evenly over the layers.

Sprinkle the remaining cheese over the top, dot with small pieces of butter, and cover with foil.

Bake covered for 25 minutes, then uncover and bake for an additional 15–20 minutes until the top is golden brown and bubbly.

Let it rest for 10 minutes before serving to allow the layers to set and flavors to blend beautifully.