In a small saucepan, whisk together remaining 1/4 cup wine and mustard. Cook over medium-high heat, whisking constantly, until mixture is thickened and smooth, about 3 minutes. Add mustard mixture to syrup; whisk to combine. Transfer mixture to a bowl. Let cool to room temperature (about 1 hour). Refrigerate and use within 1 week or can for longer storage. (See below.)

 

If you want to preserve the mostarda for longer, ladle mostarda into hot, sterilized canning jars, leaving 1/2 inch of space at top. Wipe off rims of filled jars with a clean, damp kitchen towel, then top with lids and screw on “finger tight” (just screwed on with your fingertips, not cranked tight with your palm). Put sealed jars on rack in canner or pot and add enough water to cover by 2 inches. Bring to a boil, covered, then boil 45 minutes. With tongs, transfer jars to a towel-lined surface to cool for 12 to 24 hours. After jars have cooled, press center of each lid to be sure its concave, then remove screw band and lift jar by lid by your fingertips. If lid stays put, it has a good seal. Replace screw band. Put any jars that haven't sealed properly or are not filled to within 1/2 inch of top, in refrigerator and use first.