Preheat the oven to 390°F (200°C). Line a baking sheet and spray it generously with oil; spread the panko breadcrumbs on the sheet and toss with 1 teaspoon taco seasoning.
Bake the seasoned panko for 5–7 minutes until golden, stirring or shaking the tray once to ensure even toasting. Remove from the oven and increase the temperature to 430°F (220°C).
In a shallow bowl whisk the egg, then stir in the flour, mayonnaise, Dijon mustard, remaining 1 teaspoon taco seasoning and ½ teaspoon salt to form a thick batter.
Place a chicken breast between two sheets of baking paper or cling film and flatten with a rolling pin or meat mallet until about ½ inch (1.25 cm) thick; repeat for all breasts.
Set a wire rack over a baking sheet and lightly spray or brush with oil. Dip each flattened chicken breast into the batter, then press into the toasted panko on the tray so both sides are well coated. Transfer the coated breasts to the oiled wire rack.
Bake the chicken at 430°F (220°C) for 10 minutes until crispy and golden.
Meanwhile, mash the peeled and pitted avocado with a pinch of salt using a fork until slightly chunky.
When the chicken is done, switch the oven to broil (high). Top each schnitzel with smashed avocado, then evenly sprinkle beans, chopped red bell pepper, chopped tomato, chopped red onion and cilantro over each piece. Add about ¼ cup shredded cheese on each.
Broil for 2–4 minutes until the cheese melts and turns golden. Serve immediately.
