They won't even know (or care) that they are eating vegan with this amazing Tempeh, Lettuce, and Tomato Sandwich with all the bells and whistles.
In a large skillet, heat avocado oil to medium. Add onions and saute for 2-3 minutes. Add mushrooms and garlic and saute until the mushrooms soften - 2-3 more minutes. Add balsamic vinegar and salt and pepper to taste. Remove from pan with a slotted spoon to a paper-towel lined plate.
Heat the oil in the pan to medium-high temp and add the slices of tempeh. Cook until they are a deep golden brown (3-4 minutes), then turn the slices and cook until the other sides are also a deep golden brown. Remove from pan and set to the side.
Toast 8 slices of bread. Spread 1/2 tbsp vegan mayonnaise on each slice. Top four slices of bread with the cheese slices. On top of cheese, add lettuce, tomato, 2 ounces of grilled tempeh, onion/mushroom mixture, and sliced avocado. Top with second slice of bread. Cut in half and enjoy!
Toasting the bread is optional, but it helps keep the sandwich all together!