Preheat oven to 180°C fan forced. Place taco shells onto a tray and into the oven for 5 minutes or until warm.
Heat oil in a non-stick frying pan over medium-high heat. Add mushrooms and Mexican seasoning. Cook, stirring, for 5 minutes or until soft. Add chicken and carrot. Cook, stirring, for 3 minutes, adding a little water if mixture is too dry. Remove from heat.
Combine the avocado and tomatoes in a bowl, squeeze over the lemon and season to taste, stir gently to combine.
Spoon chicken mixture into warm taco shells. Sprinkle with cheese, top with lettuce and a dollop of avocado and tomato mixture. Serve.