Thaw the frozen prawns, drizzle with 1 tablespoon of lemon juice and season with salt and pepper. Let stand for 30 minutes.
In the meantime, whisk the egg yolk, salt and 1 tablespoon lemon juice well and gradually add the olive oil and keep stirring. When the mixture is nice and creamy, fold in the pressed garlic and season with salt and pepper. Serve the finished mayonnaise with the prawns.