Wash, dry, and cut chicken into small pieces. Mix mustard, tomato paste, balsamic vinegar, salt, and pepper in a bowl. Place chicken in the marinade, refrigerate, and let it marinate for at least 1 hour.
Halve the avocado, remove the stone, and mash with a fork. Mix in lemon juice, 1 finely chopped red onion, 1 minced garlic clove, and 2 diced tomatoes. Season with salt, pepper, and chilli to taste.
Heat oil in a pan over medium-high heat. Fry the marinated chicken for 6-8 minutes until cooked through (internal temperature 74°C/165°F). Remove chicken and set aside.
Add the remaining red onions (cut into rings) to the hot pan. Fry for 3-4 minutes until golden. Add the grated cheese, stir for 1 minute, then remove from heat.
Cut baguette lengthwise. Butter one half and rub with 1 minced garlic clove. Arrange the 4 cheese slices on the other half. Place both halves on a baking sheet and bake at 190°C (375°F) for 4-5 minutes until cheese melts.
Spread avocado cream on the buttered half. Layer the cooked chicken on top, then add the remaining sliced tomatoes and the onion-cheese mixture. Place the cheese-covered half on top and serve.