Cook pasta according to packet instructions.
Meantime, drain tuna oil into a fry pan. Use this oil to quickly cook onion/s until just translucent. Throw in garlic. You can either throw in the additional vegetables now or, if they don't need much cooking, you can toss in with pasta at next step.
When pasta is done, throw it in into the fry pan with the onion, garlic, and vegetables.
Add lemon juice and toss, ensuring pasta is well mixed with tuna oil, lemon juice and vegetables.
Turn down heat and add tuna, ensuring it is spread throughout the pasta. If you wish to add a little parmesan (or butter, or chilli) for taste you can do so.
Add parsley, season with salt and pepper.