Slice the tempeh in half lengthwise, then slice each piece in half to create 4 small, thin pieces.
In a zip-top bag, combine tempeh, soy sauce, black pepper, and mustard seeds. Seal and toss to coat. Marinate for 30 minutes, flipping halfway.
Preheat oven to 425°F (220°C). Heat olive oil in a large skillet over medium-high heat. Cook tempeh in batches until golden crisp, about 3-5 minutes per side.
Spread butter on each slice of rye bread. Toast on a baking sheet in the oven for 3-5 minutes.
Spread cashew cheese (if using) on the bottom bread slices. Layer with cooked tempeh, sauerkraut, and Swiss cheese. Return to oven for 5 minutes to melt cheese.
Meanwhile, make the dressing: whisk tahini, lemon juice, chopped pickles, and pickle juice in a small bowl. Add water, 1 tbsp at a time, until smooth. Season with salt and pepper.
Spread dressing on the inside of the remaining bread slices. Remove sandwiches from the oven, add the top bread slice (dressing-side down). Slice in half and serve immediately.