Place the prawns, oil and lemon juice in a shallow dish and toss together, set aside to marinate. For the aioli, crush the garlic and sprinkle over the salt, rub to a smooth paste with the edge of a knife, set aside. Whisk the egg yolk and mustard together in a large bowl. Slowly, drop by drop, whisk in the olive oil until it is all added and a smooth mayonnaise is formed. Stir in the garlic, lemon juice and plenty of ground black pepper.