Char the corn: Heat a large, heavy-bottom skillet over high heat until very hot. Add oil (1 tablespoon), then spread the corn (3 bags) and red onion (1/2 cup) in an even layer.Let sit undisturbed for 2–3 minutes, then stir occasionally and cook for 5–7 minutes total, until kernels develop deep golden-brown charred spots.Season lightly with a small pinch of salt if desired (optional). Add aromatics: Reduce heat to low. Add garlic (2 cloves) and jalapeño (1) and cook for 1 minute, just until fragrant. Remove from heat and let cool to room temperature.
Build the salad base: In a large bowl, combine the cooled corn mixture, tomatoes (2 cups), and radishes (4-5).
Make the dressing: In a small bowl, whisk together mayonnaise (1/2 - 3/4 cup), sour cream (1/4 cup), lime zest and juice, chili powder (or Tajín) (1 teaspoon), smoked paprika (1/2 teaspoon), cayenne (1/4 teaspoon) (if using), black pepper (1/2 teaspoon), sugar (1/4 teaspoon), and cilantro (1/2 cup).
Assemble: Add half of the cotija cheese (5 ounces) to the salad. Add just enough dressing to lightly coat and gently toss to combine.
Taste and adjust: Taste before adding salt. Cotija can be quite salty, and depending on the brand you use, you may not need any additional salt. Adjust with more lime juice or dressing as needed. (You may have some dressing leftover).
Finish and serve: Transfer to a serving dish. Top with remaining cotija, pepitas, and fresh cilantro.
Optional (recommended): Let the salad sit for 10–15 minutes before serving to allow the flavors to meld.