Wash and clean the mussels thoroughly. Peel onion and cut into strips. Wash tomatoes and cut into slices. Prepare pasta in boiling salted water according to package instructions.
Heat the oil in a pot. Sauté onion strips in it until translucent. Add mussels, wine and stock and steam for about 5 minutes in a closed pot. Drain the mussels and throw away the closed mussels.
Pour the pasta into a sieve. Mix tomatoes, pasta, pesto and mussels. Arrange on a plate.