1) Blend the mint, basil, pine nuts and garlic in a food processor until finely chopped. With the machine running, gradually add the olive oil, processing until well blended. Transfer the pesto to a medium bowl. Stir in the Parmesan and season to taste with salt and pepper.
2) Toss the prawns with the olive oil in a large bowl to coat. Sprinkle with salt and pepper, and toss again.
3) Heat a large, heavy skillet over high heat. Working in two batches, add the prawns and sauté until just cooked through, about 3 minutes.
4) Toss the prawns with enough pesto to coat.
Transfer to a platter and serve.