HEAT oil in large cast iron skillet on medium-high heat. Add onions and garlic; cook and stir 3 min. or until crisp-tender.
ADD meat; cook 10 min. or until done, stirring frequently.
MEANWHILE, whisk egg and milk in medium bowl until blended. Add corn muffin mix, peppers, 1/2 cup cheese and 1 tsp. taco seasoning mix; stir just until muffin mix is moistened.
DRAIN meat mixture; return to skillet. Add tomato sauce, oregano and remaining taco seasoning mix; stir until blended.
SPOON corn muffin batter over meat mixture; spread to within 1 inch of edge of skillet.
BAKE 25 to 30 min. or until top is golden brown and toothpick inserted into center of cornbread mixture comes out clean. Remove from oven.
TOP with remaining cheese; let stand 5 min. before serving with the WHEAT THINS for dipping.