Prepare the Vegetables: Finely chop the bell peppers and onions. You can use a food processor for quicker results, but be careful not to over-process them into a paste.

Combine Ingredients: In a large pot, combine the chopped bell peppers, onions, vinegar, sugar, and salt.

Cook the Relish: Bring the mixture to a boil over medium-high heat, stirring occasionally. Once it reaches a boil, reduce the heat to low and let it simmer for about 30 minutes, stirring frequently. The relish should thicken slightly and the vegetables should become tender.

Sterilize the Jars: While the relish is simmering, sterilize your canning jars and lids by boiling them in water for 10 minutes. This step is crucial for safe preservation.

Can the Relish: Carefully ladle the hot relish into the sterilized jars, leaving about 1/2 inch of headspace at the top. Wipe the rims clean, place the lids on, and screw on the bands until fingertip-tight.

Process the Jars: Process the filled jars in a boiling water bath for 10 minutes to ensure they are sealed properly. Start timing once the water returns to a full boil.

Cool and Store: Remove the jars from the water bath and let them cool on a towel or cooling rack. Once cooled, check the seals by pressing down on the center of each lid. If it doesn't pop back, the jar is sealed. Store sealed jars in a cool, dark place. Unsealed jars should be refrigerated and used within a few weeks.