Preheat the oven to 180°C (350°F).
Cook the penne pasta in a large pot of boiling salted water according to package instructions until al dente. Drain and set aside.
In a large saucepan, heat the olive oil over medium heat. Add the chopped onion and sauté for 5 minutes until softened.
Add the minced garlic and cook for 1 minute until fragrant.
Stir in the butter and allow it to melt.
Sprinkle the plain flour over the onion mixture and stir continuously for 2 minutes to form a roux.
Gradually whisk in the milk, ensuring no lumps form. Cook for 5 minutes until the sauce thickens.
Add the canned chopped tomatoes, dried oregano, dried basil, salt, and black pepper to the sauce. Stir to combine and cook for 5 minutes.
Remove the sauce from heat and stir in half of the grated cheddar cheese until melted.
Add the drained tuna and cooked pasta to the sauce. Gently fold everything together until evenly combined.
Transfer the mixture into a 9×13 inch (23×33 cm) baking dish.
Sprinkle the remaining grated cheddar cheese evenly over the top.
Cover the dish tightly with aluminum foil.
To freeze: Place the covered dish in the freezer for up to 3 months.
To cook from frozen: Preheat the oven to 180°C (350°F). Remove the foil and cover with fresh foil. Bake for 50 minutes, then remove the foil and bake for an additional 10 minutes until the cheese is golden and bubbly.
To cook from thawed: Thaw the dish overnight in the refrigerator. Preheat the oven to 180°C (350°F). Bake covered with foil for 25 minutes, then uncover and bake for 10 minutes until golden and bubbly.