Bring a large pot of water to a boil and generously season with kosher salt.
Add spaghetti and cook according to package instructions.
While the pasta cooks, transfer tuna to a large mixing bowl and break it up with a fork.
Cut butter into small pieces and add to the tuna along with black pepper.
Taste the pasta while it cooks and adjust salt if needed.
When pasta is al dente, use tongs to transfer it to the mixing bowl and toss energetically until saucy.
If it looks dry, add a splash of water or more butter.
Taste and season with salt if needed, then top with more pepper and serve immediately.
