In a large mixing bowl, combine the onion, lemon juice and salt. Reserve.
Meanwhile, bring a large pot of water to a boil for the pasta. Add salt and cook pasta to al dente.
Finely chop the parsley with the Italian seasoning to bring out the flavor of the dried herbs. Add to the bowl along with the olive oil, tuna, celery and beans and mix.
Stir in the drained pasta (hot or cold) and toss to combine.