Finely grate the lemon peel. Cook with 100 ml of water and the lemon juice for 20 minutes. Let cool down.
Roughly grate the pumpkin and mix with the sugar. Stir over medium heat and simmer for 4 minutes.
Add the mustard flour, Dijon mustard and salt to the lemon stock and stir to a smooth cream. Add to the pumpkin, stir without stopping and cook over high heat for 1 minute. To taste.
Fill the jars to the brim, screw them on and stand on the lids to let them cool down.