Place finely chopped cucumbers and onion in a large bowl. Sprinkle with pickling salt, toss well, and let sit for 1–2 hours.
Drain mixture thoroughly in a colander and rinse lightly under cold water. Drain again to remove excess moisture.
In a large saucepan, combine vinegar, water, sugar, garlic, mustard seeds, and black pepper. Bring to a boil.
Add drained cucumber mixture and fresh dill to the saucepan. Simmer gently for 5–7 minutes.
Ladle hot relish into sterilized jars, leaving ½ inch headspace.
Remove air bubbles, wipe rims clean, and apply lids and bands fingertip tight.
Process jars in a boiling water bath canner for 10 minutes (adjust for altitude if needed).
Remove jars and allow to cool undisturbed for 12–24 hours. Check seals before storing.
Store in a cool, dark place for up to 1 year. For best flavor, wait 1–2 weeks before opening.
