Wash and dice the cucumbers into small pieces. Finely chop the onion and mince the garlic.

In a mixing bowl, combine the diced cucumbers, chopped onion, minced garlic, and sea salt. Mix well and let sit for 10 minutes to draw out moisture.

In a separate container, dissolve the sugar in the filtered water. Add the black peppercorns, mustard seeds, and dill seeds.

Transfer the cucumber mixture into a fermentation jar. Pour the brine over the cucumbers, ensuring they are fully submerged. Seal the jar and leave it at room temperature for 1-2 weeks to ferment.

After fermentation, store the relish in the refrigerator. It can be enjoyed immediately but develops more flavor over time.