Chop 2 cups of white onion and 6 cloves of garlic into small pieces. Use a sharp knife to ensure uniform sizes for even cooking.

Combine the chopped onion and garlic in a saucepan. Add 2 cups white wine, 1 cup white wine vinegar, 1 teaspoon salt, 4 peppercorns, and 1 tablespoon fresh rosemary. Cook over medium-high heat uncovered for about 20 minutes until the onion is soft and translucent.

Pour the cooked mixture through a wire mesh strainer into a bowl, discarding the solids. Ensure all flavorful liquid is collected by working in batches if necessary.

Return the strained liquid to the saucepan. Stir in 1 cup yellow mustard seeds and 1/3 cup dry mustard until fully blended.

Cover the saucepan and let the mixture sit at room temperature for at least 24 hours to allow the mustard seeds to soften and release their flavors.

Wash and inspect six 4-ounce jars, ensuring they are free from chips and cracks. Sterilize the jars by placing them in the canner filled with water and heating over medium heat. Warm the lids in a small saucepan with water on low heat to soften the adhesive.

Transfer the mustard mixture to a food processor. Pulse while gradually adding 2 2/3 cups of water until the mustard reaches a smooth consistency.

Return the processed mustard to the saucepan. Bring to a boil over moderate heat, stirring frequently. Once boiling, reduce heat and simmer for 5 minutes.

Using a canning funnel and ladle, fill each sterilized jar with the hot mustard, leaving 1/4 inch of headspace. Remove any air bubbles by running a knife around the inside of the jar.

Wipe the rims of the jars with a damp paper towel. Place the warmed lids on each jar and screw on the rings until finger-tight.

Place the filled jars back into the water bath canner, ensuring they are covered by at least 1 inch of boiling water. Bring to a boil and process for 10 minutes (adjust for elevation if necessary).

After processing, turn off the heat and let the jars sit in the canner for 5 minutes. Remove the jars using a jar lifter and place them upright on a cooling surface to rest undisturbed for 24 hours.

After 24 hours, press the center of each lid. If the lid does not pop back, the jar is sealed. Label sealed jars with the date and store in a cool, dark place. Unsealed jars should be refrigerated or frozen and consumed within a few days.