Heat a large grill pan or cast-iron skillet over medium-high heat.
Add the corn, 1 tablespoon olive oil, ¼ teaspoon salt, and black pepper.
Spread the corn into a single layer. Cook for 6 to 8 minutes, allowing it to sit undisturbed between stirring, until lightly charred in several places.
Add the red onion during the final 3 to 4 minutes. Cook until slightly softened but still holding its shape.
Transfer the corn and onion to a large mixing bowl. Let them cool for approximately 5 minutes.
Add the halved cherry tomatoes and chopped parsley.
Add the remaining tablespoon of olive oil, lime juice, and remaining salt.
Toss gently until evenly combined.
Fold in the diced avocado last, taking care not to mash it.
Taste and adjust with additional salt, black pepper, or lime juice.
Serve immediately, at room temperature, or chilled briefly before serving.