Combine wine, vinegar, onion and garlic in a medium saucepan. Bring to a boil, then simmer for 5 minutes. Remove from heat and allow steep for an additional 10 – 15 minutes. Strain, pressing on the solids to release more liquid.
Return wine to the saucepan. Dissolve honey and salt in the liquid. Over medium heat, whisk in the mustard powder, stirring until it boils. Boil, stirring constantly, until it is thick but creamy, about 5-10 minutes (it will thicken more as it cools). Taste and adjust seasonings.
Fill hot canning jars allowing 1/4 - 1/2 inch headspace. Press the mustard down into the jar, then remove bubbles by stirring gently with a spoon handle. Add lids, then process in a boiling water bath for 10 minutes (adjusting upward for altitudes over 1000 ft). Let rest for 5 minutes in the hot water prior to removing from the canner.
Notes
4 ounces dry mustard is approximately 1 cup + 2 tsp
I am not a health professional and nutrition data is calculated programatically. Accuracy may vary with product selection, calculator accuracy, etc. Consult a professional for the best information.