Preheat the Oven: Preheat your oven to 375°F (190°C).

Cook the Onions and Potatoes: Heat the olive oil in a skillet over medium heat. Add the finely chopped onions and sauté until translucent.

Add the sliced potatoes to the skillet and cook until they start to soften, about 10 minutes.

Stir in the tomato paste and season with salt and black pepper to taste.

Assemble the Bake: Layer half of the potato mixture in a baking dish.

Top with one can of drained tuna and half of the shredded mozzarella cheese.

Repeat the layering with the remaining potatoes, tuna, and mozzarella.

Prepare for Baking: Dot the top of the layered dish with butter.

Pour the hot milk evenly over the layers.

Bake: Cover the baking dish with aluminum foil and bake in the preheated oven for 20 minutes.

Remove the foil and bake for an additional 10 minutes, or until the top is golden and bubbly.

Serve: Allow the bake to cool slightly before serving.

Serve warm and enjoy this comforting and satisfying dish.