Preheat the oven to 400°F (200°C)
Grease or line a 9x9-inch baking dish with parchment paper
In a large bowl, combine Jiffy Corn Muffin Mix, egg, milk, vegetable oil, diced green chilies, cheddar cheese, chili powder, cumin, and onion (if using)
Mix until everything is well combined, but don’t overmix
Pour the batter into the prepared baking dish and smooth the top
Bake for 20-25 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean
Let cool slightly before slicing and serving