In a bowl, combine the soy sauce and curry powder. Pour over the tenderloin and coat well.
Cover and refrigerate for one hour.
Preheat the barbecue to medium-high.
In a food processor, purée the nuts, butter, orange zest and a pinch of salt until almost creamy. Set aside.
Lower the barbecue to medium.
Season the meat with salt and pepper, and cook for 20 minutes being sure to grill all sides.
Let stand 5 minutes before thinly slicing.
On the barbecue, grill all the bread slices.
Spread the walnut butter on one side of each slice.
Arrange slices of pork on 4 slices of toast, garnish with the cheese and arugula, and top with a second slice of grilled bread.
* Roasted walnuts:
Preheat the oven to 150ºC (300ºF). Spread the walnuts on a baking sheet and bake for about 30 minutes. Stir midway through cooking time. Remove from the oven and keep at room temperature.