Bring a large pot of water to the boil and cook the pasta according to the packet instructions.
Whilst the pasta is cooking, heat a large fry pan over a medium heat.
Add the olive oil and onion, and cook for about five minutes, or until the onion is golden brown.
Add the garlic to the pan and cook for 1 minute.
Add the drained tuna to the pan, and fry for 1 minute.
Add the rinsed capers and lemon juice to the pan. Cook for a further 2 minutes, or until the lemon juice is reduced to a sticky sauce.
Stir the sour cream and parsley through the tuna, and season with salt and pepper. The sauce will look slightly dry at this stage, which is exactly how it should look.
The pasta should be ready by this stage. Reserve a large cupful of the cooking water, then drain the pasta. If the pasta is not ready, turn the heat off under the tuna sauce until the pasta is ready.
Add the drained pasta to the fry pan, and stir the pasta through the sauce. Add enough of the reserved cooking water to loosen the sauce to your liking.
Serve immediately, sprinkled with cayenne pepper. Or you can do what I do, and take the cayenne pepper to the table so everyone can add their own.