Char the poblano: Roast the poblano pepper over a gas flame, under the broiler, or in a dry skillet until charred all over and the skin is completely blackened. Transfer to a bowl and cover for 5 to 10 minutes. The trapped steam helps loosen the skin so it peels off easily.
Prep the poblano: Once cool enough to handle, use your fingers or a paper towel to gently peel off and discard the skin, remove the seeds, and finely dice the pepper. Don't rinse the pepper, as that removes the smoky charred flavor.
Cook the corn: If using frozen corn, add it to a large dry skillet over high heat and cook for 5 to 10 minutes until thawed and lightly charred. If using fresh corn, shuck the corn and carefully slice off the kernels. Add the kernels to a dry skillet over high heat and cook for 5 to 10 minutes until lightly charred, stirring occasionally.
Mix the salsa: Add the charred corn, roasted poblano, jalapeño, red onion, cilantro, lime juice, and salt to a bowl. Stir well to combine.
Chill and serve: For the best flavor, refrigerate until chilled before serving.
