Put two saucepans of water on to boil. Trim off the woody stems of the asparagus and cut the spears into 2.5 cm (1 in) lengths. Trim the green beans and cut them in half. Slice the courgettes thinly. Trim and cut the leeks into quarters lengthways, then into 1 cm (½ in) slices.
Put the asparagus and beans into one pan of boiling water and cook them for 4 minutes, or until just tender. Drain, refresh them with cold water and set aside.
Add the pasta to the remaining pan of boiling water and cook according to the instructions on the packet.
Meanwhile, heat the olive oil in a wok or heavy-based nonstick frying pan over a medium-high heat. Add the garlic and stir-fry for 30 seconds, then add the courgettes and leeks and stir-fry for 3 minutes, or until the courgettes are tender but not too soft.
Add the asparagus and beans to the wok or pan and stir-fry for 1 minute, then mix in the lemon juice and tarragon. Remove from the heat and season with salt and pepper to taste.
Drain the pasta, reserving the cooking water. Stir the pasta into the vegetables, using a few teaspoons of the cooking water to moisten, if necessary. Sprinkle with the parsley and extra black pepper and serve.