To make the caramelized sugar, heat sugar in an ovenproof (1 litre, 4 cups, 1 3/4 pints) plan mould, over low heat, stirring constantly. Cook until sugar has melted and is golden brown. Tilt pan to coat bottom and sides. Remove from heat and let cool for at least 30 minutes, tilting pan occasionally to coat sides. To make the custard, mix half of the milk with 3 tablespoons sugar and peel of 1 lemon. Cook slowly over low heat for 1/2 hour, stirring constantly. Set aside. Remove lemon pieces and discard. In large bowl beat whole eggs and egg yolks lightly. (Egg shells can be saved and added later to water of the Baño María to prevent from splashing into flan while cooking). Blend in 3 tablespoons of sugar and remaining milk. Gently add the boiled milk and stir. Pour custard mixture into mould on top of caramelized sugar. Place mould in pan of hot water (Baño María). The water should cover more than half of mould. Cook on top of stove over a medium flame for 30 minutes. transfer to a preheated F (180 C) oven and continue cooking for another 30 minutes or until an inserted knife comes out clean. Immediately remove from water and cool, covered, at room temperature. Refrigerate, covered, until ready to serve. Un mould by loosening sides of custard with a knife and place a serving plate over mould. Turn over on to plate.