Remove lemon peel with fine grater, cook it, adding 100 milliliters of water and lemon juice for 20 minutes. Cool it. Grate the pumpkin on a coarse grater, mix with sugar and cook over low heat for 4 minutes. Mix the mustard seeds, Dijon mustard, salt and lemon mixture until the cream, add the pumpkin and cook, stirring, over high heat for 1 minute. Season to taste.