Preheat the oven to 375 and grease a 2-1/2 quart casserole dish with a nonstick cooking spray.: As the oven warms to 375 degrees, you will notice a dry warmth build that primes the ceramic dish to brown the edges of the casserole. Greasing the dish prevents sticking and helps the sides release cleanly when serving, yielding pretty portions. If you skip adequate greasing, the edges can cling and tear when scooped, so be thorough and coat the dish evenly.
Add vegetable oil, diced onion, minced garlic, salt, dried parsley and cayenne pepper to a large skillet or cast iron pan.: As these aromatics hit the warm pan, the oil will shimmer and you will smell the onion releasing a sweet steam while the garlic begins to swell with scent. This initial sizzle develops the savory foundation for the casserole, which is why I prefer a skillet with even heat distribution. A common mistake is adding garlic too early and allowing it to brown too fast, which can introduce bitterness.
Heat seasonings and onion over medium high heat, until onions are tender and translucent in color.: You should watch the onion go from opaque to a glossy, pale color, and you may hear a gentle sizzling. This transformation softens the texture and deepens the sweet notes, which balances the corn. If the pan gets too hot and the onion crisps or burns, lower the flame and stir more often to avoid acrid flavors.
Remove skillet from heat and stir in drained whole kernel corn and 1 can of creamed corn plus chopped jalapenos. Set skillet aside.: Off the heat, the mixture will cool slightly and the corn juices will mingle with the softened onion, producing a sweet, steamy aroma. Stirring in the jalapenos now preserves their freshness and bright bite. If you add them earlier while the pan is searing, you might lose some bright pepper flavor, so this timing keeps the heat lively but balanced.
Add dried Jiffy cornbread mix and 2 eggs to the prepared casserole dish.: When the dry mix and eggs meet in the dish, you will notice the mixture thicken and begin to hold shape as you stir. The eggs provide structure and help the cornbread mix integrate into a batter that sets when baked. Avoid overmixing at this stage, which can develop a denser texture rather than a tender custard.
Stir in skillet jalapeno corn mixture, sour cream, and melted butter. Mix well.: Once combined, the batter should look glossy and slightly lumpy from the corn kernels. The sour cream enriches and adds a tang, while melted butter brings silkiness. Stir gently so you maintain pockets of whole kernel texture. A frequent error is adding too much liquid or over stirring so the mixture becomes soupy, which can prevent a proper set in the center.
Sprinkle top of corn casserole with grated Parmesan cheese.: The grated Parmesan Cheese will rest on the surface, where it melts into a savory crust and helps form a golden, slightly crisp layer. When it starts to turn color you will notice a nutty aroma that signals readiness for the oven. If you use too much cheese, it can form an overly hard top, so aim for an even, moderate layer.
Bake for 25 minutes.: As the casserole bakes, you will hear a faint bubble at the edges and see the top firm slightly while staying soft in the center. The heat causes the eggs to set and the cornbread mix to finish its bind. Ovens vary, so check visually for a set center; if the middle remains very wobbly after the time, it needs more baking rather than removal.
Remove fiesta casserole from the oven and top with panko and butter mixture.: Lifting the dish reveals steam and a fragrant, toasty aroma. The Panko Crumbs mixed with melted butter should be spread evenly to create that satisfying crunch. If you delay adding the topping, it may soak in instead of crisping, so apply it promptly while the casserole is hot for best texture contrast.
Continue to bake the side dish for an additional 20 minutes or until the top is golden brown. Oven times may vary as different oven temps vary as well as elevations of cooking. Corn casserole is done when the center is set.: During this second bake, the Panko Crumbs will toast and the entire surface will take on a warm, golden hue and emit a toasty fragrance. The center should be set when you gently jiggle the dish. A common issue is overbrowning the top while the center remains underdone; if that happens, tent loosely with foil and bake a bit longer to finish the interior without burning the crust.
Remove from oven and serve hot. Optionally drizzle with sweet honey.: When you lift the casserole, steam will rise and the aroma will intensify, signaling it is time to serve. A drizzle of honey can add a bright sweet contrast to the savory, spicy elements if you like a sweet finish. Allowing it to rest for a few minutes helps the center firm for cleaner slices; serving immediately can lead to a looser center that slides but still tastes wonderful.
