olive oil
butter
onion
salt
sugar (optional)
garlic
tomato paste
tomatoes
basil
Parmesan rind
white wine or water as needed
To serve
dried pasta such as spaghetti or linguine
chili flakes
Parmesan
salt and freshly ground black pepper to taste
Heat butter and olive oil in a large pot until the butter melts and starts foaming. Add the diced onion, salt and sugar and cook, stirring, over low heat for 5 minutes. You want the onion to soften but not color.
Turn the heat to as low as possible, cover the pot and cook for a minimum of 30 minutes (and up to 90 minutes), stirring once in a while. Add a splash of water or white wine whenever the pot looks dry, especially if cooking for longer. Discard the cheese rind and basil before using.
Prepare the pasta (preferably spaghetti, linguine or buccatini) per package instructions. Drain, reserving some of the pasta water. Add the sauce to the drained pasta and toss together to combine. Stir in a splash of the pasta water if needed.
Drain the tuna and fold in the pasta. Check the seasoning and adjust, if needed. Serve with freshly grated Parmesan, a pinch chili flakes and my garlic dough balls.