In a large mixing bowl, combine finely chopped cucumbers, red bell peppers, and onions. Sprinkle the mixture with salt and stir well. Let this sit for 1 hour.

After 1 hour, transfer the vegetable mixture into a colander to drain off the liquid. Gently press down to remove excess moisture.

In a non-reactive pot, combine sugar, white vinegar, apple cider vinegar, mustard seeds, celery seeds, turmeric, and chili flakes. Bring to a boil, stirring until sugar dissolves.

Carefully add the drained vegetable mixture to the pot. Stir and let it come to a boil, then reduce heat and simmer for 10 minutes.

Ladle the hot relish into sterilized jars, leaving ½ inch headspace. Wipe rims and secure lids tightly.

If canning, process in a boiling water bath for 10 minutes. If not, cool completely and store in the refrigerator.