In a large pot of boiling salted water, cook the farfalle until al dente, about two mins before the package cooking time.
In the meantime, blanch the asparagus in a large pot of boiling water for 2 mins, until they turn bright green.
Have a large bowl of ice water ready.
Reserve ⅓ cup of cooking liquid and drain the asparagus. Immediately transfer to ice water for 1 minute to stop the cooking process. Cut the spear off and reserve for later.
Transfer the asparagus stems into a food processor, add salt and pepper to taste and 1 tbsp of oil. While blending all ingredients, slowly add the liquid preserved until you reach a creamy consistency.
Add prawns in a bowl,drizzle with 1 tbsp olive oil, salt and pepper to taste. Mix well until evenly seasoned.
Heat a large grilling pan over med-high heat. When hot, add the prawns and grill 1-2 mins on each side.
Drain the pasta and transfer into a large salad serving bowl. Add the sparagus sauce, the reserved spears, grilled prawns.
Grate fresh lemon zest over the pasta salad, mix gently all the ingredients and serve at room temperature or chilled.
Store in an airtight container for up to 1 day in the fridge.