Heat a large dry non-stick skillet over medium high heat. Note: you can add baking spray to the pan to prevent the tempeh from sticking. Layer the tempeh flat in the pan so it doesn't overlap, and cook for about 4 minutes on each side until browned, carefully flipping with a fork so as not to break the tempeh pieces. 

Meanwhile, make the spicy mayo: In a small bowl, add the vegan mayo, Sriracha and lime juice. Stir until combined. Place the sauce in the fridge while you wait for the tempeh to cook so it doesn't thin out while it sits. 

When the tempeh is warmed and ready, toast the bread in your toaster as desired. Cut 2 tempeh pieces in half and place the 4 pieces in one direction across one piece of bread. Then cut the other 2 pieces of tempeh in half, and layer over the other tempeh, going in the other direction (see step by step photos above). 

Add the tomato, avocado, and spinach leaves. Then finally, spread 1 tablespoon of spicy mayo on the other piece of bread, and top the sandwich. Cut down the middle and enjoy.