Split the vanilla pod lengthwise and scrape out the seeds with a small knife.
Crush the seeds with a bit of the brown sugar to separate them.
Crush the fennel seeds in a mortar and chop the dill.
Mix the homemade vanilla sugar, brown sugar, the two types of mustard, crushed fennel seeds, finely chopped dill, and apple cider vinegar in a bowl.
Stir well until the brown sugar is almost dissolved.
Now whisk in the oil a little at a time until the sauce becomes shiny and smooth.
Season with lemon juice, salt, and pepper, and the fox sauce is ready to serve with the salmon.