Bring a medium saucepan of salted water to the boil for the spaghetti. Trim the base of the fennel, halve lengthwise and thinly slice using a sharp knife or mandoline. Finely chop the parsley leaves, discarding stems. Finely chop the garlic, then using the flat side of the knife, mash to a paste (see cooking tip). Finely grate the zest of half the lemon*, then juice the half.
Step 2 — Make parsley oil
Combine the parsley, one-quarter of the garlic, 2 tsp lemon juice and 1½ tbs extra virgin olive oil in a bowl and season with salt and pepper.
Step 3 — Cook spaghetti
Add three-quarters of the spaghetti* to the pan of boiling water. Cook, stirring occasionally, for 12 mins or until al dente. Reserve 180ml (¾ cup) cooking water, then drain the spaghetti.
Step 4 — Cook fennel
Meanwhile, heat 1 tbs olive oil in a large frypan over medium-high heat. Cook the fennel, stirring occasionally, for 5 mins or until softened. Add the Tuscan seasoning, remaining garlic and ½ tsp chilli flakes* (you can omit the chilli if you prefer) and cook, stirring, for 1-2 mins until fragrant.
Step 5 — Cook tomatoes and prawns
Add the tomatoes to the fennel and cook, stirring, for 1 min or until slightly blistered. Add the prawns, lemon zest and 125ml (½ cup) of the reserved cooking water. Cook, covered, for 2-3 mins until the prawns are just cooked through and the sauce is reduced.
Step 6 — Get ready to serve
Stir 2 tsp lemon juice into the prawn mixture. Add the spaghetti, then gradually add the remaining cooking water, stirring constantly, until the sauce coats the spaghetti and looks slightly glossy. Taste, then season with salt and pepper. Divide the spaghetti among bowls. Drizzle with the parsley oil to serve.