Soak the seeds overnight, or for at least two hours, in the wine and vinegar. I use a homemade vinegar that started its life as Sauvignon Blanc. Meanwhile, toast the peppercorns in a small pan over medium high heat until they are fragrant (a couple minutes). Remove to a mortar and grind until fine with a pestle. Adding the kosher salt can help break down the peppercorns. Seal in a baggie with the mustard powder, and brown sugar.
Next day put all ingredients into a blender and puree to the desired consistency. Don’t be alarmed if the mixture looks too thin. It will thicken with a bit more blending and time! Transfer to a glass jar and seal. It will keep in the refrigerator for several months.