Preheat oven to 200°C (180℃ fan forced). Cook the pasta in well salted boiling water until just al dente. It will continue cooking in the oven, so don’t overcook. Drain and set aside.
In a large pan, melt the butter over medium heat. Add the crushed garlic and cook for 20 to 30 seconds, stirring, until fragrant. Keep the heat moderate so it doesn’t burn.
Add the flour and stir constantly for 1 minute to form a smooth paste. This removes the raw flour taste and helps create a thick, creamy sauce.
Gradually pour in the milk, whisking as you go to keep the sauce smooth and lump free. Continue stirring for 3 to 5 minutes until the sauce thickens enough to coat the back of a spoon. Season with salt and pepper.
Stir through about half of the cheese until melted. Add the drained tuna and cooked pasta, then mix well so everything is evenly coated in the sauce.
Transfer to a baking dish and spread out evenly. Sprinkle over the remaining cheese, then bake for 12 to 15 minutes until hot and lightly golden on top.