Here's one I invented a lot of years ago for a vegetarian friend. Not low fat, but it is good (what can I say, I'm originally from California).
Lay the bread slices on a flat surface. Put half the avocado slices on each bread half. Top one bread half with the remaining ingredients, dressing this mess VERY LIGHTLY with oil and vinegar. Top with remaining avocado/bread half, and cut into quarters to serve.
For a non-vegetarian version of this, I've added thin slices of smoked turkey.
I've also made this by piling the whole mess into halves of pita bread, but if you can get good whole-grain bread, that's even better.
Kathy in Bryan, TX