Finnish Mustard

We almost always had a jar of this in the house: my mother loves her mustard. This is wonderful — so quick to make and it will keep well for a few weeks in a glass jar in the fridge. You could find it becomes essential for cold meat sandwiches, roast ham and with smoked sausages and gravadlax. I imagine it would be nice with any mustard powder you use, but this is the way my mother always makes it.