Combine the wine, vinegar, water, onion, and garlic in a small saucepan. Bring to a gentle simmer and simmer for 5 minutes.
Strain out and discard the onion and garlic, reserving the liquid. Stir the honey into the warm liquid until dissolved.
Allow the liquid to cool to lukewarm.
Whisk the ground mustard and salt together in a small bowl.
Sift the ground mustard mixture into the liquid a little at a time, whisking thoroughly after each addition until smooth and fully combined.
Return the saucepan to low heat. Cook, stirring regularly, until the homemade Dijon mustard has reduced to just shy of the desired consistency. Do not cook it to its final thickness; it will continue to thicken as it cools and rests.
Remove from the heat and cool completely to room temperature. Transfer to a clean container, cover, and refrigerate for at least 8 hours before using. For the best flavor and consistency, refrigerate for 24 hours before serving.
Notes
Use a dry white wine that you enjoy drinking; avoid cooking wine and sweet varieties.
Fresh ground mustard will give the finished Dijon a stronger, more pronounced mustard flavor than an older container that has been sitting in the pantry.
If the mustard tastes too sharp after it has rested, stir in a little additional honey, adding it gradually to taste.
This is intentionally a small-batch recipe. It can be doubled, but avoid increasing it further because the longer reduction time can cause the mustard to develop a bitter flavor.