Place the peeled and halved russet potatoes in a medium saucepan. Fill with enough water to cover the potatoes. Add some salt, cover and bring to a boil. When the water is boiling, turn it down to medium low and allow to simmer in the water until the you can stick a knife easily into the potatoes. Drain.
Add the butter, milk and ground pepper into the saucepan with the potatoes. Using a potato masher or a fork, mash the mixture until smooth. Set aside.
Heat some oil in a skillet on medium heat. When the oil is hot, cook the onion until transparent. Add in the garlic, and cook until fragrant. Add in the tuna, and mix in. Add some salt and pepper to taste. Remove from heat.
Preheat the oven to 400°F.
Press the tuna mixture on the bottom of an 8×8 baking pan. Layer the mashed potatoes on top of the tuna mixture. Sprinkle the cheese on top.
Place the pan in the preheated oven for 15 minutes, or until the cheese is melted.