Start with cooked, cooled rice ready; the total time does not include cooking rice. Preheat the oven to 350°F (175°C). Grease a 9×13-inch baking dish with butter or spray.
Mix the dry: In a large bowl, combine cornmeal, salt, baking soda (and baking powder if using), onion, and jalapeño. Stir to mix evenly.
Whisk the wet: In another bowl, whisk together eggs, milk, oil, and cream-style corn until smooth.
Combine: Pour the wet mixture into the dry and stir just until no dry spots remain.
Fold in rice: Gently fold in the cooked rice so it’s evenly distributed.
Add cheese: Fold in the cheddar, reserving a small handful for topping if you like.
Pour & smooth: Spread batter evenly into the prepared baking dish. Sprinkle reserved cheese on top.
Bake for 45–50 minutes, until golden, the center is set and reaches 160°F, and a toothpick in the center comes out clean.
Cool & serve: Let rest 10–15 minutes before slicing into squares. Serve warm.