Preheat oven to 350°F. Line a tray with parchment paper. Cut each chicken fillet in half horizontally, cutting almost all the way through. Open out the fillets so they sit flat. Place each fillet between 2 sheets of plastic wrap and pound with a rolling pin or meat mallet to flatten. Set aside.
Whisk the egg and milk in a shallow bowl or dish. Set aside. Break up taco shells and place in a food processer with parmesan cheese. Process until fine crumbs. Transfer to a plate. Combine flour and Taco spice mix on a sheet of baking paper.
Working with 1 chicken fillet at a time, add to the flour mixture and turn to coat. Dip in the egg mixture, draining off the excess, then coat in the parmesan crumb mixture, pressing to coat well. Transfer to the prepared tray.
Heat about 1 cm of oil in a large non-stick frying pan over medium-high heat. Cook the chicken, in batches, for 2-3 minutes each side or until golden and crisp. Transfer to a plate lined with paper towel to drain.
Meanwhile, combine the tomato, taco sauce and garlic in a small saucepan over low heat. Simmer, stirring often, for 5-6 minutes, until slightly thickened. Remove from heat.
Place the chicken schnitzels in a baking dish. Pour over the tomato mixture and sprinkle with the cheese. Bake for 15-20 minutes or until golden and the cheese is melted. Serve on top of a bed of French fries, topped with avocado, sour cream, parsley or coriander, and chilli.