In a large skillet over medium heat, saute the onion in the olive oil until golden. Add the walnuts. After about a minute, add the salmon and then the wine. Allow it to evaporate. Add salt and pepper to taste.
Meanwhile, cook the pasta until al dente. Drain, retaining some of the cooking water.
Place the pasta in a large serving bowl. Mix in the salmon, with a little of the pasta water if it seems dry. Mix in the mozzarella.