Put the cream, lemon zest and juice into a saucepan and bring to the boil. Boil for 2 minutes. Because of the lemon, you should now have quite a thick mixture.
Boil the pasta in salted water until just al dente.
Heat the butter in a sauté pan and gently cook the onion until soft, then add the garlic and cook for another minute. Stir in the crab and warm through. Season to taste.
Cook the peas and beans in boiling water for 3-4 minutes, then drain.
Drain the pasta and add it back to the pan in which it has cooked. Quickly reheat the cream and combine the pasta with it, then add the vegetables and the crab mixture along with the chopped parsley. Toss gently and taste for seasoning. Serve immediately