Step 1

Shell and devein prawns, leaving tails intact. Reserve shells and prawn heads. Peel 3 garlic cloves, then thinly slice; bruise remaining unpeeled cloves. Quarter onion, leaving skin intact. Remove rind from lemon with a zester; squeeze juice from lemon (you will need 2 tablespoons juice).

Step 2

To make prawn stock, heat 1 tablespoon of the oil in a large saucepan over a high heat; cook reserved prawn heads and shells, stirring, until changed in colour. Add onion and parsley stalks; cook, stirring occasionally, until browned lightly. Stir in the water; bring to the boil. Reduce heat to low; simmer, uncovered, for 15 minutes. Strain stock through a fine sieve into a heatproof jug or bowl; discard solids. Return liquid to same cleaned pan; simmer, uncovered, for 20 minutes or until stock has reduced to 1 cup (250ml).

Step 3

Meanwhile, cook pasta in a large saucepan of boiling salted water until just tender; drain. Return pasta to pan with 2 tablespoons of the remaining oil; toss to combine. Cover to keep warm.

Step 4

Heat 1 tablespoon of the remaining oil in a medium saucepan over high heat. Cook prawns, turning, for 2 minutes or until changed in colour and just cooked through. Remove from pan.

Step 5

Heat remaining oil in small saucepan over medium heat; cook garlic and chilli, stirring, for 1 minute until golden brown. Remove with a slotted spoon; drain on paper towel. Add parsley leaves to pan; cook for 10 seconds (be careful as it will splatter). Stir in rind.

Step 6

Add parsley oil to spaghetti in pan with prawns, garlic, chilli, stock and juice; toss well. Season to taste. Serve immediately.