Place the chicken breast in a pot. Pour enough water so that the chicken is fully submerged into the liquid.
Bring the water to a boil. Reduce to a simmer and continue cooking the chicken until it is no longer pink and cooked through, about 20 minutes.
Shred the chicken once it is cool enough to handle (long shreds are ideal).
Place the shredded chicken into a bowl, and add the mayonnaise and finely chopped onions. Mix thoroughly.
Season with salt, pepper and sugar.
Assemble the sandwich:
Take a slice of wheat bread and spread 1½ spoonfuls of the chicken mixture on the surface.
Arrange the tomato slices and the lettuce on top.
Take another slice of the bread and spread 1 spoonful of the chicken mixture on one side.
Place the bread on top of the layer of tomatoes and lettuce, chicken spread side down. This will "glue" the second layer onto the first.
Spread another 1½ spoonfuls of the chicken mixture on the other side of the second layer.
Place the ham and the cheese slices on top.
Take the last slice of wheat bread and spread 1 spoonful of the chicken mixture on one side. Place the bread on top of the layer of ham and cheese, chicken spread side down.
Slice the assembled sandwich diagonally into half. Serve.
Notes
I find that boiling chicken breast with its skin and bones on helps retain moisture. This means a juicier and tastier chicken spread.
I often boil my chicken with aromatics such as ginger, green onions or both, but this is optional.
Once cooked, break the chicken into big chunks with your hands to let it cool faster.
Recipe by The Gourmand's Table at https://thegourmandstable.com/a-hefty-clubhouse-sandwich/