Whisk together 2 teaspoons of the lime juice, the olive oil and a little pepper in a small bowl. Add the onion slices and toss to coat. Set aside to allow the onion to mellow in flavour while you prepare the remaining ingredients.
Roughly mash the avocado with a fork. Add the remaining lime juice and the Tabasco or chilli sauce, to taste, and carry on mashing until the mixture is fairly smooth. Mix the mayonnaise with the coriander.
Preheat the grill or a ridged chargrill pan to medium. Place the bread on the grill tray or pan and toast or chargrill for 2–3 minutes on each side until lightly browned.
Spread two slices of toast very thinly with some of the mayonnaise, then spread with half the mashed avocado, dividing it equally. Top with the chicken and onion slices. Spread another two slices of toast very thinly with mayonnaise, then place over the chicken filling, mayonnaise-side down. Spread half the remaining mayonnaise thinly over the tops of the sandwiches. Spread with the remaining avocado, then add a layer of sliced tomatoes and the sprouts.
Finally, spread the last of the mayonnaise over the last two slices of toast and place on the sandwiches, mayonnaise-side down. Press the sandwiches together, then cut each in half. Serve immediately.