Preparation

1. Fill a large stock pot with water and bring to a boil. Add corn ears to the stock pot. Bring back to a boil and cook for 3 minutes. Place cooked ears of corn into a bowl of ice water to stop the cooking process. Pour cooking water from the stock pot and set the pot aside for later.
2. Cut corn kernels from the cob and transfer to a large bowl. Toss corn with the diced onion, bell pepper, and (if using) the jalapeño pepper.
3. In the large stock pot, add apple cider vinegar, granulated sugar, mustard seed, celery seed, cumin, red pepper flakes, kosher salt, and freshly cracked black pepper. Bring to a boil, medium-high heat.
4. Add corn mixture and bring to a boil. Reduce heat to low and simmer for 12-15 minutes. Use a large spoon to scrape off any foam as the mixture boils.
5. Mix cornstarch with water and add to the pot. Stir until incorporated and increase heat to medium. Continue to cook, stirring often, until the corn relish begins to thicken, 5-7 minutes.
6. Carefully transfer corn relish to prepared canning jars and cover with lids. Set jars aside to cool, using caution when handling as the mixture will be hot. Once cooled to room temperature, refrigerate the jars.