Prepare boiling water canner. Heat jars in simmering water until ready to use, do not boil. Wash lids in warm soapy water and set aside with bands.
Combine maple syrup, sugar, vinegar, water and all seasonings in a non-reactive pot and bring to a simmer for 5 minutes. Add jalapeño and onion slices, simmer 5 minutes.
Ladle hot jalapeños and onions into a hot jar leaving a ½ inch headspace; top off with more brine if necessary. Remove air bubbles. Wipe jar rim. Center lid on jar and apply band, adjust to fingertip tight. Place jar in boiling water canner. Repeat until all jars are filled.
Process jars 15 minutes, adjusting for altitude. Turn off heat, remove lid, let jars stand 5 minutes. Remove jars and cool 12-24 hours. Check lids for seal, they should not flex when center is pressed.
*Wear gloves when working with hot peppers