How to Make It
Place saffron threads in a small skillet; cook over low, swirling skillet constantly, until threads are dry and toasted, about 1 minute and 30 seconds. Crumble toasted saffron into a small bowl; add 2 tablespoons hot water, and stir to combine. Let steep 10 minutes.
Bring a large pot filled with salted water to a boil over high. Add asparagus; cook, stirring often, until vibrant green, about 1 minute. Using a spider, remove asparagus, and plunge into a bowl filled with ice water; let stand 3 minutes. Drain and set aside. Add spaghetti to boiling water in pot; cook, stirring occasionally, until just al dente (about 2 minutes less than cook time specified in package directions). Drain and reserve 1 cup pasta cooking liquid.
While spaghetti cooks, heat oil in a large, deep skillet over medium until shimmering. Add shallot, garlic, and crushed red pepper; cook, stirring constantly, until slightly softened, 1 to 2 minutes. Add asparagus, prawns, and saffron-water mixture; cook, gently turning prawns occasionally, until light pink in spots, about 2 minutes. Add vermouth; bring to a boil. Transfer prawns to a plate using a slotted spoon; continue boiling mixture in skillet, undisturbed, until liquid is slightly reduced and alcohol flavor cooks off, 3 to 4 minutes.
Add cooked spaghetti to asparagus mixture in skillet; toss to coat using tongs. Return prawns to skillet; add butter, parsley, tarragon, and chives. Increase heat to medium-high; cook, stirring and swirling skillet constantly, adding reserved cooking liquid as needed (1/4 cup at a time) until a creamy sauce forms and coats noodles, about 3 minutes. Season with salt to taste. Serve immediately.
Spot prawn season runs from February through October. Spot prawns are available online from catalinaop.com or from your local fishmonger. If unavailable, substitute jumbo sweet white shrimp from the Gulf Coast.