Preheat oven to 400 degrees.
Wash and slice squash. Dice yellow onion.
In skillet over medium heat, add a little oil or butter. Add squash and onion. Season with salt and pepper. Cook until squash is softened, about 10 minutes. Turn off heat.
In a large mixing bowl, prepare your cornbread mixture by adding the box of Jiffy, milk, and egg. Once those ingredients are mixed, add in the sour cream and combine. Add cheese and jalapeno at this point if desired.
Add the cooked squash and onion to the cornbread mix. Stir to combine.
Pour the mixture back in the cast iron skillet. Carefully place skillet in oven using oven mitts as it will still be hot.
Bake in pre-heated oven until a fork or toothpick comes out clean from the center. About 30 minutes but can vary depending on the size of your skillet/pan.