Preheat grill or broiler. Bring 3 cups water to boil in a small saucepan, and set up an ice bath nearby.
In a mixing bowl, stir together the cooked beans, rosemary, 2 tablespoons of the olive oil, red onion, marjoram, and the juice and zest of lemon. Season the mixture with salt and pepper.
Plunge the mint leaves into the boiling water for 30 seconds and remove to ice bath. When the mint leaves have cooled, remove them from the bath, and gently squeeze to remove any excess liquid. Place the mint and parsley leaves in a food processor and process for 1 minute with 3/4 cup extra-virgin olive oil. Season with salt and pepper. Set aside.
On a large plate, arrange the beans in 1 pile and the mache drizzled with oil in another pile.
Season the prawns with salt and pepper, brush with oil, and grill until opaque and cooked through, about 7 minutes.
Remove the prawns from the grill and stack them in another pile on the plate.
Drizzle the dish with the mint oil and serve immediately.