Whisk the muscovado sugar and lemon juice together until the sugar has dissolved.
Add the mustard, oil and mayonnaise, whisking all the time.
Finally, add the fresh dill, season with a pinch of seasalt and freshly ground pepper. Whisk again. This sauce is meant to be fairly runny. If you find it too runny, add a little more mayonnaise. (If you have a blender, put all the ingredients in and whizz for one minute!)
This sauce can be kept for a week in the fridge and actually tastes even better a day old, as it allows the dill to penetrate the sauce even more.