In a large deep pan over medium heat add 1 tbsp oil. Add onion & pepper and fry until soft and just starting to pick up colour. Add in sun dried tomatoes & garlic and fry for another couple of mins.
Pour in tomato passata, then add in basil, chilli flakes, a pinch of sugar (to balance out the sourness of the tomatoes) and a good pinch of salt and pepper. Give it a stir, then reduce to a low simmer for 20mins, stirring occasionally. This is important to marry the flavours together.
About 8mins before the sauce is done, pop your pasta in salted boiling and cook until al dente (take a min or so off cooking, you want it a touch hard).
Combine drained pasta with sauce, then add tuna chunks. Gently toss in the tuna (don't be too aggressive or the tuna turns to mush).
Add half of the pasta to a baking dish, followed by a layer of mozzarella, followed by the rest of the pasta, then finish with a layer of cheddar. Bake in the oven at 200C/390F for 15-20mins or until golden and bubbly.
Serve with any leftover basil, then tuck in and enjoy!