The BLT is a sandwich I feel strongly about. There is a right way to do it. And while many would insist that way involves bacon, according to Paul, the B stands for beer, which is absolutely what you should serve this sandwich with.
Apart from beer, you need good crusty bread. A soft crusted sandwich loaf will absolutely not work here. There is a deli near my house that sells prepackaged vegetarian BLTs on whole wheat sandwich bread with yellow mustard and cheddar cheese. Despite its proximity to my house and even its other reputable vegan offerings, I cannot patronize this establishment.
Just before serving, you slather two slices of that good bread (lightly toasted if not super fresh) with your favorite vegan mayonnaise (which is vegenaise, of course). The act of slathering is important here. Your finished BLT should ooze and drip. This is not a polite sandwich.
On top of one slice of slathered bread you place several slices of beautiful ripe tomato. This is a summer sandwich. Make it now while tomatoes are juicy and sweet. You should be eating a BLT at least once a week until the tomatoes run out.
On top of the tomato you will arrange your slices of smoky braised tempeh. The order matters. The mingling of tomato juice, mayonnaise, and then the sweet, smoky, saltiness of the tempeh forms a sort of essential dressing within the sandwich.
Now on the other half of bread mash a quarter of a soft, ripe, oily avocado. You can, of course, make a BLT without avocado, but it's always better with. Take one big billowy leaf of lettuce - the sort with a nice little crisp rib near one edge - and fold it in half. (I know an even more reputable sandwich shop that once used a handful of soft, peppery mixed baby greens for the L. I don't eat there anymore either. The pliable crunch and mild fresh flavor of leaf lettuce is a supporting, though essential, element of the BLT.) Gently press the lettuce into the avocado.
Place the lettuce and avocado half on top of the tempeh and tomato half and devour. Often.
[A quick note on the kid-friendliness of this sandwich - the five year old will not touch it, but the two year old is a sandwich-stealing super fan.]