Preheat & prep pan: Preheat your oven to 425°F (200°C). Line a 12-count muffin tin with liners or coat with cooking spray.
Mix dry ingredients: In a large bowl, whisk together 1 cup yellow cornmeal, 1 cup all purpose flour, 3 tablespoons granulated sugar, 2 teaspoons baking powder, and 1/4 teaspoon kosher salt.
Mix wet ingredients: In a separate bowl, whisk together 1 egg, 1/4 cup buttermilk, 2 tablespoons melted butter, and 1 can creamed corn until well combined.
Combine: Pour the wet mixture into the dry ingredients and stir with a spatula until just combined; do not overmix. Fold in 1 cup shredded sharp cheddar or Monterey Jack cheese, 2 finely diced jalapeños, and 1/2 cup finely diced yellow onion.
Bake: Spoon batter into muffin liners, filling all the way to the top. Top with the remaining 1/2 cup of cheese and sliced jalapeño rounds. Bake for 5 minutes 425°F then, keeping the muffins in the oven, reduce heat to 350°F and continue baking for another 16–17 minutes. The total time these muffins should be in the oven is about 21–23 minutes. Use a toothpick to test. If it comes out clean, the muffins are done.
Serve: Let cool for 10 minutes before enjoying.
