Finely dice the bell peppers and onion to a uniform 1/4-inch size. Transfer the chopped vegetables to a large, non-reactive pot or Dutch oven.

Add the apple cider vinegar, sugar, mustard seed, celery seed, and salt to the pot. Stir well to combine all ingredients.

Bring the mixture to a boil over medium-high heat. Once boiling, reduce the heat to low and maintain a steady simmer for 30-40 minutes, stirring occasionally. The relish is done when the liquid has reduced and thickened to a syrupy consistency.

For refrigerator storage, allow the relish to cool before transferring it to jars. It will keep for up to 3 weeks. For long-term storage, proceed with water bath canning.

Ladle the hot relish into hot, sterilized half-pint jars, leaving 1/2-inch headspace. Wipe rims, apply lids and rings until fingertip-tight. Process in a boiling water bath for 10 minutes. Let jars rest in the canner for 5 minutes before removing to cool.