Heat a large skillet over medium-high heat. Slice the onion, then add it to the skillet along with the ground beef and ground turkey. Break the meat apart with a spoon and cook for 8-10 minutes, stirring occasionally, until browned.
Add 1 can of Rotel tomatoes (or 1 cup diced tomatoes), 0.5 teaspoon cumin, 0.5 teaspoon chipotle chili powder (or 1 teaspoon chili powder), 0.5 teaspoon oregano, and 2/3 cup water. Stir well to combine.
Reduce heat to medium and simmer the meat mixture uncovered for 8-10 minutes, stirring occasionally, until slightly thickened.
While the meat cooks, prepare the toppings: chop the tomatoes into bite-sized pieces, slice the avocados, chop the onion, roughly chop the cilantro, roughly chop the black olives, and tear or chop the salad leaves.
Preheat oven to 175°C (347°F). Place the corn taco shells on a baking sheet and bake for 5 minutes until warmed and crisped.
Place the warm taco shells on a serving plate.
Fill each taco shell with the meat mixture, sprinkle with taco cheese, and top with salsa sauce, sour cream, tomatoes, avocado, onion, cilantro, olives, and salad leaves.