Combine the cashews, milk, mustard, sauerkraut juice, vinegar, salt, and pepper in a blender.
Process until smooth and then pour into a shallow dish.
Stir in the scallion, flour, and baking powder.
Heat ⅛ inch of oil in the skillet over medium-high heat. (These are prone to sticking, so add additional oil if needed.)
Dip the bagels into the mixture, let the extra batter drip back down into the dish, and transfer to the skillet.
Cook for 3 to 4 minutes on one side, or until browned.
Cook the other side the same way.
Place a bagel half on each plate and spread evenly with the avocado mixture.
Divide the seitan/sauerkraut mixture evenly among the bagels.
Put the tops on and serve.