Wash all vegetables thoroughly and finely chop them using a food processor.

Transfer the chopped vegetables to a colander, sprinkle with salt, and let sit for 1-3 hours to drain excess water.

Rinse the vegetable mixture under cold water and squeeze out excess liquid.

In a large pot, combine the squeezed vegetable mixture with apple cider vinegar, sugar, turmeric, celery seeds, mustard seeds, and red pepper flakes (if using).

Bring the mixture to a boil, then reduce heat and simmer uncovered for 20-25 minutes while stirring frequently.

Taste and adjust seasoning if necessary.

Cool the relish completely and spoon into sterilized jars, sealing tightly.

Refrigerate for at least 24 hours before serving.