1Preheat your oven to 375°F (190°C).
Peel and slice the potatoes into thin rounds.
In a large pot, bring water to a boil and cook the potato slices for about 10 minutes, or until they are just tender. Drain and set aside.
In a saucepan, melt the butter over medium heat. Add the milk and stir until well combined.
In a baking dish, layer half of the potato slices on the bottom.
Spread half of the drained tuna over the potatoes.
Pour half of the milk and butter mixture over the tuna and potatoes.
Add half of the chopped tomatoes on top.
Repeat the layers with the remaining potatoes, tuna, milk mixture, and tomatoes.
Bake in the preheated oven for 25-30 minutes, or until the top is golden and bubbly.
Remove from the oven and let it cool for a few minutes before serving.