In a small bowl, whisk together the flour, water, sugar, black pepper, and salt. Add the panko breadcrumbs to another bowl.
Dip each piece of tempeh into the flour batter and slightly shake it to remove the excess batter. Transfer immediately to the bowl of panko and flip the tempeh to coat it evenly on all sides. Transfer to a plate.
Heat about 1 cup of neutral oil over medium heat in a deep saucepan or skillet. Once the oil is hot, add the panko-coated tempeh and fry for 2-3 minutes. Flip once halfway through frying or until golden brown.
Remove from the oil by using a fork or a slotted spoon and transfer to a plate lined with kitchen paper towels.
Curried Mayo
In a small bowl, combine the mayo, chili sauce, and curry powder. Stir until fully combined. At this point, you can taste and adjust the spiciness to your liking by playing with the ratio of mayo to chili sauce.
To Assemble
Steam your buns according to the package instructions (usually 12-15 minutes).
Open the buns and arrange about 1/4 cup of lettuce and a few leaves of cilantro inside each bun. Top with the katsu tempeh and generously drizzle with the curried mayo.
Top with the finely chopped peanuts and shredded red cabbage if using. Serve immediately!
Notes
*You can chop the peanuts with a sharp knife or process them for a few seconds in a food processor.