Cook the pasta in salted water until al dente, drain and keep warm.
Meanwhile, heat the oil in a large pan to medium heat. Sauté the onion and celery for 8 minutes until soft and tender brown. Add the zucchini and garlic and stir-fry for a minute or two. Add the drained beans, parsley, tomatoes, the zest of the lemon, its juice and the vegetable stock to the pan, season with pepper / salt and the chilli flakes. Heat for a few minutes and remove the pan from the heat. Mix the vegetable mixture with the pasta. Arrange immediately on plates, sprinkle with parsley and serve drizzled with olive oil if desired.