Way to ferment mustard seeds for homemade mustard

Dijon has a strong taste from the liquid they use in it (it's not quite vinegar, not quite wine). I'd probably try a blend of some other mustard + a little white wine vinegar (not white vinegar; white wine vinegar) or champaigne vinegar or apple cider vinegar. (Maybe even a dry white wine, if you have that on hand).

If you have dry mustard powder, and some time to let it sit, you could also try making your own; most of the recipes online seem to use a dry white wine as their base, a few use white wine vinegar.

You really need to add the oil slowly at first, especially if whisking. You are trying to disperse the fat into tiny droplets suspended in the water phase (the water from the butter and the vinegar). You can't whisk fast or hard enough, especially by hand, to break up 3.7 parts fat in 1.3 parts water when all the fat is trying to combine into a single mass floating on the water, no matter how much mustard you have.

I would have whisked the mustard into the vinegar, then slowly added the butter, then the oil.