Begin by preparing the vegetables. Wash and peel the yellow squash, then finely chop it. Also, finely chop the onions, red bell pepper, and green bell pepper.
In a large bowl, combine the chopped yellow squash, onions, and both types of bell peppers. Sprinkle with salt and allow them to sit for 30 minutes. This will help draw out excess moisture.
After 30 minutes, drain the vegetables in a colander and rinse them under cold water to remove excess salt. Drain thoroughly.
In a large pot, combine the drained vegetables, sugar, apple cider vinegar, mustard seeds, celery seeds, turmeric, cinnamon, cloves, and red pepper flakes. Stir well to combine.
Place the pot over medium-high heat and bring the mixture to a boil. Reduce the heat to low and let it simmer for about 30 minutes, stirring occasionally until the relish thickens and the vegetables are tender.
Season the relish with salt and black pepper to taste. Adjust the sweetness and spiciness according to your preference by adding more sugar or red pepper flakes if needed.
Once the relish has reached your desired consistency and flavor, remove it from heat and let it cool for a few minutes.
Transfer the yellow squash relish into clean, sterilized jars with airtight lids. Seal the jars while the relish is still hot to create a vacuum seal.
Allow the jars to cool at room temperature, and once they are cool, store them in the refrigerator. The relish will continue to develop its flavors over time.
