Pre-heat oven to 180C/350F.
Cut tempeh in half horizontally to create two large, thin sheets. Cut each sheet into 6 even pieces.
Place tempeh pieces in a single layer in a baking dish and cover evenly with the marinade ingredients. Place baking dish in the oven and allow to bake for 30 minutes or so.
Slice onions and throw them into a saucepan with the oil and salt. Slowly cook over a low-medium heat until caramelised and golden brown (about 20 minutes).
Combine all Russian dressing ingredients in a small bowl and mix well.
Place bread slices under the grill and toast one side until golden. Flip bread over, place cheese on top of each slice and grill until cheese melts.
Remove from grill and place a generous dollop of Russian dressing on each cheesy toast slice.
On 6 of the bread slices, assemble sandwich by placing two pieces of tempeh on each bread slice, topped with a slice or two of tomato, a large dollop of sauerkraut and caramelised onions.
Top with another cheesy toast slice and wrap your laughing gear around it!