Heat the butter in a pan. Add the bell pepper, zucchini and cauliflower florets. Sprinkle with salt and pepper. Cook over medium heat, stirring occasionally, for about three minutes.
Add the onion and garlic and cook, stirring often, for another half a minute.
Pour in the yogurt. Stir. Taste and add more salt and pepper, as needed.
When the sauce is simmering, add the cooked pasta. Cook, tossing, until the pasta is heated through and has soaked up the sauce.
Sprinkle with parsley and nuts before serving.