Cook base. In a large pot, combine onions, wine, vinegar, salt, garlic, peppercorns, and rosemary. Bring to a boil over medium-high heat, then reduce to simmer for 15-20 minutes until onions are soft.
Strain and add mustard seed and powder. Strain mixture through fine-mesh strainer into large bowl, pressing solids to extract liquid. Discard solids. Add mustard seeds and mustard powder to strained liquid. Cover and let stand 24-48 hours.
Prep for canning. Wash jars and place in water bath canner with water. Simmer to keep hot. Wash lids and rings, set aside.
Blend. Transfer mustard mixture to high-speed blender. Blend starting slowly, then medium speed until paste forms. Add 1 cup water, then slowly add more water until desired consistency is reached.
Heat Transfer to saucepan and bring to boil over medium heat, stirring constantly. Reduce heat and simmer 5 minutes.
Fill jars. Ladle hot mustard into hot jars, leaving 1/2 inch headspace. Remove air bubbles, wipe rims clean. Apply lids and rings fingertip tight.
Process in canner. Process in boiling water bath for 10 minutes. Remove from heat, let sit 5 minutes, then transfer jars to cool overnight. Let cool undisturbed until jars seal.