Bring a large pot of salted water to a boil. Add the husked corn and cook for about 3 minutes, just until the kernels are crisp-tender.

Immediately transfer the corn to a bowl of ice water to stop the cooking. Drain the ears well and pat them dry before cutting off the kernels.

Use a serrated knife to cut downward along each cob over a shallow bowl or rimmed pan. This catches the kernels and makes the cutting process easier to manage.

Whisk the olive oil, apple-cider vinegar or lemon juice, kosher salt, and black pepper in a small bowl until combined. Taste the dressing and use the larger amount of acid if you prefer a sharper salad.

Toss the corn and red onion with the dressing while the corn is still slightly warm. Add any optional tomatoes, cucumber, jalapeño, cilantro, or feta at this point.

Fold in the basil immediately before serving so the leaves stay green and aromatic. Serve at room temperature or chill the salad for 20 to 30 minutes to let the flavors develop.