Marinate the tempeh in a mixture of soy sauce, maple syrup and apple cider vinegar. Let it marinate for at least 15 minutes.
Put the thinly sliced red onion in a bowl and cover with vinegar and sugar. Leave it in the refrigerator for at least 30 minutes.
Heat some oil in a frying pan over medium-high heat. Fry the marinated tempeh slices until golden brown and crispy on both sides. Remove them from the pan and drain them on paper towels.
Spread one side of each slice of bread with (vegan) mayonnaise.
Top two slices of bread with slices of fried tempeh, pickled onion, mixed lettuce, tomato slices and avocado slices. Season with salt and pepper. Place the 2nd slice of bread on top.