Cook the pasta al dente according to the instructions on the packet.
In the meantime, cut the leek into thin slices. Let the olive oil get hot in a pan and sauté the leek in it. Pour in the vegetable stock and add the cheese to the pan and let it melt.
Drain the tuna well in a sieve and then chop it up well with a fork. Season the sauce with salt and pepper and stir in the drained tuna. Simmer for 5 minutes over low heat. Pour the sauce over the finished pasta.