Using a mandolin, thinly slice cucumbers and onion. Layer them with some fresh sprigs of dill in empty clean ball jars.
Make the brine. In a small saucepan over low heat, mix together vinegar and sugar until the sugar dissolves. Add celery seeds, mustard seeds and salt. Stir to combine.
Remove from heat and let cool.
Pour brine into the jars until just covering the top layer of cucumber. Seal the jars and place in the refrigerator. Give the flavors a week to meld before enjoying.