Wash jars, lids and screw bands in hot, soapy water, then rinse well. Dry screw bands. Put jars on a rack in canner or deep pot and add enough water to cover by 2 inches. Put tongs and ladle into pot, making sure lower 5 inches of tools are submerged in the water. Bring to a boil and boil for 10 minutes (see note).

Remove from heat, leaving jars in water. In a small saucepan, heat lids in water to cover by 2 inches until thermometer registers 180° (do not let boil). Remove from heat, leaving lids in water. Keep the jars and lids submerged in hot water, covered, until ready to use.

Bring a large pot of water to a boil. Fill a large bowl 1/3 full with ice and water. Blanch apricots 30 seconds, then, using a slotted spoon, transfer to prepared ice bath. Drain apricots, then peel, pit and cut into 1/2-inch-thick slices.

In a medium saucepan combine sugar and 2 cups water; bring to a simmer over medium-high heat. Add apricots, return to simmer until cooked through but firm, about 1 minute. Remove from heat.

In a small saucepan, whisk together mustard powder and 1/2 cup water. Over medium-high heat, bring mixture to a simmer, whisking until smooth, then add mixture to pan with apricots and stir well to combine.

Drain jars upside down on a clean kitchen towel 1 minute, then invert. Using a slotted spoon, divide apricots among jars, then cover with syrup, leaving ½ inch of space at top. Discard any excess syrup. 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 20 minutes. With tongs, transfer jars to a towel-lined surface to cool. Jars will seal; if you hear a ping, it signals that vacuum formed at the top of cooling mostarda has made lid concave (some jars make the sound after you remove them from water, and others in same batch may take a few hours); the important thing is for jars to eventually have concave lids.

After jars have cooled 12 to 24 hours, press center of each lid to check that it’s concave, then remove screw band and lift jar by lid with your fingertips. If lid stays put, it has a good seal. Replace screw band. Put any jars that are not filled up to a 1/2 inch from top, or any that haven’t sealed properly, in the refrigerator and use them first. Mostarda can be used immediately, or store properly sealed jars in a cool, dark place, up to 1 year.

NOTE:  Add 1 minute of processing time (both to sterilization and final processing) for every 1,000 feet above sea level.