Preheat the oven to 375 degrees F (190 degrees C).
Heat a large skillet over medium-high heat. Add ground beef, and cook, stirring to crumble, until no longer pink, about 10 minutes. Remove meat with a slotted spoon and set aside. Drain off most of the fat, leaving enough to coat the bottom of the pan. Add onions to the fat in the pan; cook and stir until tender, about 5 minutes. Return the meat to the skillet and season with garlic powder, salt, pepper, cumin, chili powder and oregano. Mix in jalapeno slices and cook until heated through. Transfer this mixture to an 8 inch square baking dish.
Meanwhile, in a medium bowl, stir together the cornbread mix, butter, milk and egg. This can be done while the meat is cooking. Spoon over the top of the meat mixture in the baking dish, then top with shredded cheese.
Bake in the preheated oven until the top is golden brown and a knife inserted into the center comes out clean, about 40 minutes. Cool slightly, then cut into squares to serve.