Remove the fine skin from the peanuts. Make small piles of peanuts on a large sheet of nonstick baking paper, spacing them well apart and then pressing down on them so that you get small circles about 2cm in diameter.
Pour the sugar into a pan, add 4 tablespoons of water and cook over a low heat until you get a golden caramel. Add the lemon juice, mix and take off the heat. Pour the hot caramel over the peanuts very carefully so that you get little round biscuits. Leave to harden and then unstick the peanut crunch from the baking parchment and serve.
Never stir the caramel as it cooks as this will introduce air. While it is cooking, you can also brush the sides of the pan with a damp pastry brush to prevent the sugar strands from caramelising.