Put the mustard powder, starch and sugar in your pan. Stir/massage with a whisk so there are no clumps.
Add the cream and mix evenly. The consistency will be quite thick. Heat while stirring, so no clumps form and it does not burn. Let the mustard boil gently for around 4-5 minutes.
Take your pot off the stove and add the vinegar. Pour into a clean, sterilized jar. Let cool, and transfer to the fridge to mellow at least for overnight before serving.
Notes
This amount makes exactly one cup of ready mustard. As extra flavoring you can add 1 tablespoon of cognac.The mustard stores for 2 weeks in the fridge.