Place butter, onion, celery and carrots into your Instant Pot and press “Saute”.
Cook for 2 minutes, until veggies turn softer.
Adjust with salt and pepper and add tuna.
Cook for another 2 minutes.
Add tomato sauce, pasta and water and mix a bit.
Close the lid and cook at high pressure for 8 minutes.
When cooking time ends, do a quick pressure relesae.
Pour your cooked pasta into a baking tin, sprinkle with mozzarella and grill in the oven for a few minutes till the cheese melts.
Serve and enjoy!