Combine the chopped onion, chopped garlic, dry white wine, white wine vinegar, salt, black pepper corns and rosemary sprig in a large stainless-steel or enamel saucepan.
Bring to a boil over high heat; reduce heat, and simmer, uncovered, 15 to 20 minutes or until the onion is soft and translucent, stirring occasionally.
Remove the sauce pan from heat and pour the onion mixture through a mesh strainer into a glass or stainless steel bowl. Discard solids.
Stir the yellow mustard seeds and dry mustard into the onion liquid in the bowl. Cover and let stand at room temperature at least 24 hours, but no longer than 48 hours.
Process the mustard mixture in a blender or food processor, adding the water until the consistency of cooked oatmeal.
Transfer the mustard to a small stainless steel or enamel saucepan. Bring to a boil, stirring often; reduce heat and simmer, uncovered, 5 minutes.
Ladle the hot mustard into a hot jar, leaving ¼ inch headspace. Remove air bubbles. Wipe the jar rim. Center a flat lid on the jar. Apply the band and adjust to fingertip tight. Place jar in the boiling water canner. Repeat until all jars are filled.
Process the jars 10 minutes, adjusting for high altitude. Turn off heat, remove the lid, and let jars stand 5 minutes. Remove the jars and cool.
Makes 6-8 four ounce jars.