In a small bowl, mix the mayonnaise and mustard together.
Heat the griddle to medium. Arrange the bacon in a single layer on the griddle and cook, flipping occasionally, until crisp, 8 to 10 minutes. Transfer to a paper towel-lined plate to drain.
While the bacon is cooking, toast the bread on the other side of the griddle, until lightly browned, about 1 minute per side.
Spread about 2 teaspoons of the mayo mixture on one side of each slice of bread.
Top two of the bread slices with avocado, season lightly with salt and pepper, then add the bacon, tomato and lettuce, dividing everything evenly between the two sandwiches. Top with the second slice of bread, mayo side down.
Slice each sandwich in half and serve immediately.
Notes
No griddle?--I like using my Blackstone griddle for cooking the bacon and toasting the bread since it's convenient to be able to cook them at the same time, but you can use a cast iron skillet for cooking the bacon. Pop the bread in a toaster or brown it over the grill for 30 to 60 seconds, if desired.
Slice it cleanly--Use a sharp serrated knife for cutting your sandwich so that all of the fillings slice cleanly.
Storage--Store leftover sandwiches in your cooler or fridge, wrapped in foil or plastic wrap, and enjoy within 1 day.