In a bowl, put chopped banana peppers, green bell pepper, red bell pepper, onion, garlic, and pickling salt.
Stir everything with a spoon and cover the bowl with plastic wrap.
Leave it for 1 to 2 hours.
After 2 hours, remove all the liquid.
In a big pot, mix apple cider vinegar, white sugar, and mustard seeds. Heat until it boils.
Put the pepper mixture into the pot and turn down the heat.
Let it cook gently for 25 to 30 minutes.
Turn off the heat and let it cool.
Put the relish into a jar and cover it with a lid.
Place the jar in a hot water bath canner.
If the jar is 8 oz, process it for 10 minutes. If it's 16 oz, process for 15 minutes.
After canning, let the relish rest for 7 days to pickle.
After 7 days, it will be ready to use.