Boil the Potatoes: Place the peeled and cut potatoes in a pot.

Cover with cold water, add 1 teaspoon salt, and bring to a boil.

Reduce heat and simmer for 15-20 minutes or until potatoes are fork-tender.

Drain and let cool slightly.

Prepare the Onion and Tuna: While the potatoes cook, slice the onion into thin half-rings.

Drain the oil from the canned tuna and mash it with a fork in a bowl.

Assemble the Casserole: Preheat oven to 200°C (390°F).

Grease a baking dish with olive oil.

Layer the Ingredients: Place half of the sliced potatoes as the base layer in the dish.

Add a layer of sliced onions over the potatoes.

Spread the mashed tuna evenly over the onions.

Sprinkle half of the shredded mozzarella cheese over the tuna.

Make the Cream Sauce: In a medium bowl, whisk together heavy cream, lemon zest, and minced garlic.

Season with salt and black pepper to taste.

Complete the Layers and Bake: Pour the cream sauce evenly over the casserole.

Add the remaining sliced potatoes and sprinkle with the rest of the shredded mozzarella cheese.

Place a sprig of fresh rosemary on top for added fragrance (optional; remove before serving).

Bake in the preheated oven for 25-30 minutes or until the cheese is melted, bubbly, and golden.

Serve: Let the casserole cool slightly before serving. Enjoy warm with your favorite side dish or salad.