Place the corn kernels in a bowl and allow them to thaw Or, for faster thawing, put the corn in a colander and rinse with cool water until thawed. Allow as much excess water to drain off as possible.
Dice the tomatoes and onion, and roughly chop the cilantro leaves. Place the thawed corn, tomatoes, onion, and cilantro in a bowl. Add the juice from half the lime (about 2 Tbsp), cumin, and salt. Toss until everything is evenly coated. Taste the relish and adjust the lime and salt if needed. Serve immediately or chill until ready to eat.