Heat oil in a 12-inch skillet until shimmering. Add onion; saute until soft, about 5 minutes. Add garlic, chili powder, oregano and cumin; saute until fragrant, about 1 minute. Add beef or turkey and cook, stirring often, until it loses its raw color. Stir in tomatoes; simmer to blend flavors, about 5 minutes. Add salt and pepper, to taste. Stir in cornmeal; cook, stirring constantly, until it thickens, almost instantly. Cool and refrigerate (or serve warm for dinner as tacos or taco salad).