Preheat oven to 400 degrees F.
To prepare meat filling, heat oil in 11- to 12-inch, deep, oven-proof skillet over moderate heat 1 minute. Add onions, garlic, and stir-fry mix and cook, stirring, until onions are limp and golden, 5 to 8 minutes. Add chili powder, oregano, and beef, breaking up clumps. Cover and cook 10 minutes. Blend in flour, add salsa and tomato paste, stirring well to incorporate, then add corn and simmer uncovered 5 to 10 minutes, stirring often, until meat shows no signs of pink. Season to taste with salt and pepper, mix in cilantro, and set off heat.
To prepare corn bread topping, combine flour, cornmeal, and all remaining dry ingredients in bowl and make well in center. Combine buttermilk, oil, and egg in 1-quart measure, dump into well in dry ingredients, and mix briskly just until dough holds together. Spread on top of meat filling, leaving 1- to 1 1/2-inch margin all round.
Bake uncovered until bubbly and brown, 15 to 20 minutes. Serve at once.