Combine the cucumber, green pepper, and onion in a large bowl. Pack them into a quart jar or two pint jars.
Add the cider vinegar, white sugar, celery seed, mustard seeds, and salt into a medium saucepan. Whisk to combine. Heat over medium heat, stirring until the sugar is dissolved.
Remove the brine from the heat and allow it to cool for 5-10 minutes.
Pour the brine evenly over the vegetables (distributing between 2 jars if used). This will fill about ½ to ¾ of the jars.
Pour in cool filtered water to cover the cucumbers completely, leaving about 1 inch of headspace in the jar, OR use a fermentation weight to submerge the vegetables under the brine.
Top the jars with a lid and shake them gently or turn them upside down to distribute the brine.
Chill in the refrigerator for at least 1-3 days before serving. If possible, shake the jars gently a few times during the first 24 hours.
Your finished refrigerator pickles should last 2-4 weeks when stored in an airtight container in the refrigerator. Enjoy!