In a chopper, chop the peppers, onion and garlic until slightly chunky.
Drain any excess liquid and add the chopped peppers to a saucepan over medium heat.
Add the sugar, mustard seeds and vinegar. Cook until it comes to a boil.
Take the mixture off heat and allow it to cool down completely. Transfer to a glass jar and refrigerate.
Notes
Remember to use pickling salt or sea salt for this recipe. Table salt isn't great to use for pickles and preserves.
I used a food chopper to chop up the peppers. If you prefer a chunky texture, you can use a sharp knife to manually chop up the peppers too.
Allow the relish to rest in the refrigerator for a day or more before you enjoy it. Just like with most other condiments (especially my Schezwan sauce), resting allows the flavors to deepen.
If the peppers are too juicy, you may want to squeeze and drain some of the excess liquid off after they've been chopped, and then use them for the recipe.