Place 1 (14.4 oz.) bag thawed frozen corn kernels, 1 cup thinly sliced red onion, 1/2 cup finely chopped red bell pepper, and 1 tablespoon mustard seeds in a medium heatproof bowl. Bring 1 cup apple cider vinegar, 1/3 cup sugar, 2 tablespoon canola oil, and 1 tablespoon kosher salt to a boil. Immediately pour over com mixture. Let cool to room temperature. Refrigerate in resealable jars for up to 2 weeks.