Bring a large pot of water to a boil. Generously season the water with kosher salt, add 1 lb. spaghetti, and cook according to the package instructions.
Cut butter into small pieces and add these to the tuna, along with lots of freshly ground black pepper.
Taste the pasta while it’s cooking—is it delicious already? If not, add more salt to the water. The moment it is al dente, use tongs to quickly transfer it to the mixing bowl, and begin tossing energetically until the spaghetti looks saucy. If it looks dry, you can add a small splash of water or live a little and add a few extra pats of butter. Taste and season with salt if needed. Top with more pepper and eat immediately.
Of course, you could add all sorts of things to this pasta: chopped parsley or basil, grated lemon zest, or crushed olives. Or you could serve it with lemon wedges for squeezing on top. Or you could fry up some breadcrumbs and minced garlic in olive oil for a crunchy topping.