Slice tempeh width-wise into 20-22 thin pieces. Steam sliced tempeh in a steamer basket for 10 minutes (see tips below if you don't have a steamer basket). Meanwhile, mix together the marinade: vegetable broth, soy sauce, apple cider vinegar, maple syrup, garlic powder, onion powder, smoked paprika.
Line the tempeh up in a 2-3 quart baking dish (any dish will do as long as the tempeh fits in a single layer). Pour the marinade over the tempeh. Marinate for at least 30 minutes and up to 24 hours.
Drizzle a large skillet with enough oil to coat over medium heat. Once hot, add the marinated tempeh. Reserve the marinade for later.
Saute until lightly browned on one side. Flip the tempeh. Pour in the reserved marinade. It will look like a lot of liquid, but should cook off completely within 3-5 minutes. Turn the heat up to medium-high if needed.
Continue sauteing until the bacon reaches your desired crispiness, flipping again as needed.
Make chipotle aioli: Remove one chipotle pepper from the can. Finely dice. Combine the mayonnaise, finely diced chipotle chili pepper, one tablespoon adobo sauce (from canned chipotles), lemon juice, garlic powder and salt in a small bowl. Stir until combined.
To make one sandwich: Mash ½ avocado until spreadable. Spread avocado on one piece of toasted sandwich bread. Spread 2 tablespoons chipotle aioli on the other piece of bread. To the slice with the aioli, add four slices of tomato, 5-7 slices of tempeh bacon, then ½ cup lettuce. Close the sandwich and enjoy!