This is the classic sweet mustard sauce to gravlax and crab in Sweden. It's also known under the name of hovmästarsås 'Head Waiter's Sauce', since it used to be prepared by the head waiter at the table in finer restaurants. It's a good idea to use only the dill stems when preparing the gravlax and keep the leaves for the sauce. This is my recipe:
4 tbsp sweet mild mustard (e.g. Swedish or German)
1 tbsp Dijon mustard
4 tbsp vinegar (preferably malt vinegar)
Dash of Worcestershire sauce
Dash of HP sauce
2 tbsp white sugar
1 tbsp Muscovado sugar
1 tsp salt
½ tsp freshly ground white pepper
200 ml mild vegetable oil of high quality (in Sweden cold-pressed canola oil is often used). Do not use olive oil!
Lots of fresh dill, chopped
Mix everything except oil and dill. Whisk until the sugar dissolves. Then whisk in oil, a little at a time as for mayonnaise, constantly whisking. Change the whisk to a spoon and stir in the dill (otherwise all the dill will stick to the whisk ). Adjust the seasoning.
Serve chilled with gravlax or shellfish. The sauce will keep for a few days in the refrigerator.