Reserve a few small basil leaves for garnish. Process remaining basil, almonds and garlic until finely chopped. With motor running, gradually add oil, scraping down bowl occasionally. Transfer to a bowl and stir in parmesan. Season to taste. Cover surface closely with plastic to prevent discolouration.
Cook pasta in boiling, salted water as per packet instructions. Drain pasta, reserving 1/4 cup of the cooking water. Add extra oil, prawns, pesto and lemon juice to pasta with enough of the cooking water to thin it out a little. Serve immediately, topped with reserved basil.