Combine cleaned prawns with lemon juice, salt, pepper, garlic and rosemary in a glass or other non-reactive bowl. Cover and marinate for about 1 hour in the refrigerator, stirring periodically.

Preheat a large nonstick over high heat. Transfer the contents of the bowl to the hot pan. Cook the prawns for about 2 minutes on each side, until the shells turn bright pink and the meat is opaque.

Remove to a serving bowl and set another alongside for shells.