In a small bowl, mix the wasabi powder and water together until
well blended. Whisk in the mayo, tamari, salt, pepper and garlic
powder. Set aside for a few minutes to let the flavors blend.
To assemble the sandwiches, spread 1 tablespoon of wasabi
mayonnaise on each slice of bread, then top 4 bread slices with
3 ounces of roast beef, sliced cheese, 1 lettuce leaf, tomato and
onion slices and a second slice of bread.