Rinse the spinach and watercress and place in a large saucepan with just the water clinging to the leaves. Cover and cook over a medium heat for 2 minutes, stirring and turning the vegetables occasionally, until they have wilted.
Add the peas and cook, uncovered, for 2 minutes – there should be enough liquid to cook them. Tip the greens and their liquid into a bowl. Set aside.
Cook the pasta in a large saucepan of boiling water for 3 minutes, or according to the packet instructions, until al dente.
Meanwhile, blend the cornflour to a smooth paste with the yogurt and put into the pan used for cooking the vegetables. Stir over a medium heat until it is just bubbling. Add the vegetables, parsley, basil and seasoning to taste and stir well. Heat the sauce through, then remove the pan from the heat.
Drain the pasta and add to the sauce. Toss to mix with the sauce, then serve.