Fill a large pot with lightly-salted water and bring to a rolling boil over high heat. Once the water is boiling stir in the bow tie pasta and return to a boil. Cook the pasta uncovered, stirring occasionally, until the pasta has nearly cooked through but is still firm to the bite, about 10 minutes. Drain well and rinse with cold water in a colander set in the sink.
While the pasta is boiling cook the prosciutto in a small frypan over medium heat until crisp. Drain on a paper towel-lined plate and set aside.
Make the dressing by whisking together the mayonnaise, sour cream, milk, garlic, lemon zest and juice, lemon pepper, paprika and chilli flakes; season to taste with salt and pepper.
Place the drained pasta into a large mixing bowl and pour 3/4 of the dressing over the top; reserve the remaining dressing. Add the peas, cranberries, grapes, spring onion, water chestnuts and Parmesan cheese; stir until the salad is evenly coated with the dressing. Cover and refrigerate at least an hour, or until ready to serve.
Toss the salad with the remaining dressing, oregano and thyme to serve.