1. Combine the apple cider vinegar, salt, chili flakes, and coconut sugar (use the full four teaspoons if you like your relish sweet) in a small saucepan. Bring to a boil then reduce heat to maintain a simmer for five minutes.
2. While the mixture simmers, peel the cucumber, then slice it in half lengthwise and use a spoon to scoop out the seeds before roughly chopping the cucumber flesh.
3. Chop the garlic in the bowl of a food processor then add the cucumber and pulse just until the cucumber is finely diced. Transfer the cucumber and garlic to a glass jar with a tight-fitting lid.
4. After 5 minutes of simmering remove the vinegar mixture from the heat and add the dill. Pour the warm mixture over the cucumber and garlic and press down so they are fully submerged. Cap and transfer to the refrigerator to cool at least one hour and up to 24.