Step 1

Combine white wine, water, vinegar, onion, and garlic in a saucepan; bring to a boil, reduce heat to medium low, and simmer until flavors combine, about 15 minutes. Cool to room temperature and pour through a strainer into a large bowl; reserve liquid and discard onions and garlic.

Step 2

Stir mustard seeds, dry mustard, garlic powder, and salt into strained liquid; cover the bowl with plastic wrap, and let sit at room temperature until mixture thickens, 24 to 48 hours.

Step 3

Puree mustard mixture to desired consistency using a stick blender. Transfer mustard mixture to a saucepan and add water as necessary to reach a smooth consistency. Bring mustard to a simmer, reduce heat to medium, and cook, stirring constantly, until flavors combine, about 10 minutes.

Step 4

Pack mustard into hot, sterilized jars, filling to within 1/4 inch of the top. Run a knife or thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids and screw on rings.

Step 5

Refrigerate mustard until flavors blend, at least 1 week.