Cook the noodles in a large saucepan of boiling water according to the packet instructions, until al dente. Drain, reserving 1/2 cup of the cooking water.
Meanwhile, combine the coriander, peanut butter, soy sauce, honey, vinegar, sesame oil, garlic and cayenne pepper in a food processor or blender and puree. Transfer to a large bowl.
Whisk in the reserved pasta cooking water. Add the linguine, carrots, capsicum, celery and spring onions and toss well to combine the ingredients. Chill the noodles in the refrigerator for at least 1 hour before serving.
The pasta cooking water, which carries some of the pasta's starch, is used here to ‘stretch’ the sauce. It's a traditional Italian technique for thinning or smoothing a sauce so that it coats the noodles better. And the water replaces what might otherwise be a lot of extra fat.