Preheat an oven to 400°F (200°C). Oil a 9-inch gratin dish with 2-inch (5-cm) sides.
In a large frying pan over medium heat, warm 2 table-spoons of the olive oil. Add the onion and sauté until soft but not browned, 3–5 minutes. Add the bell pepper and sauté for 3 minutes longer. Add the tomatoes and cook, stirring, for another 3 minutes. Raise the heat to high and cook, stirring, until the excess moisture from the tomatoes evaporates, 1–2 minutes. Add the garlic and cook for 1 minute longer. Add the pepper flakes, if desired, salt and pepper to taste and 2 tablespoons of the basil. Taste and adjust the seasoning.

Spread half of the potatoes on the bottom of the prepared dish. Arrange half of the vegetable mixture on top and sprinkle with half of the cheese. Sprinle the tuna.  Repeat the layers. Drizzle the remaining 1 tablespoon oil evenly over the top. Cover tightly with aluminum foil. Place the covered dish on a baking sheet. Bake for 30 minutes. Remove the foil and continue to bake until the potatoes are tender and the top is nicely browned, about 15 minutes longer. Sprinkle with the remaining 2 tablespoons basil and serve immediatel.

Yield:  4-6 servings