Slice the top and tail off the beetroot (leave the skins on), then add them to a large saucepan. Fill the saucepan with cold water until the beetroot are completely submerged, then quickly bring to a boil. Continue to boil for 30 minutes, then drain.
Preheat the oven to 200°C fan / 425°F and line a small baking tray with baking paper.
Run the beetroot under cold water in a sink and - while the water is running - use your fingers to peel the skin off the beetroot. Pat dry the beetroot with a paper towel or cloth and leave to one side.
In a bowl, combine the ground black peppercorns, ground mustard seeds, coriander, smoked paprika, garlic powder and 1/2 teaspoon of salt. This is your pastrami rub.
Generously brush the beetroot all over with olive oil, then cover each beetroot in the pastrami rub. Transfer to the baking tray and bake for 30-40 minutes or until the beetroot is tender in the middle, then leave to cool for 30 minutes or so, before slicing approximately 2-3mm thick.
Meanwhile add the tofu mayo ingredients, along with a pinch of salt, to a food processor. Process until smooth, then set aside. Note: this will make more tofu mayo than you need so see our notes for storing suggestions.
To assemble the sandwiches, lightly toast the bread, then add a generous dollop of tofu mayo to the bottom slice, followed by the lettuce leaves, beetroot pastrami slices and sauerkraut. Spoon some extra dijon mustard on the top slice, then add the slice on top and cut in half to serve. Enjoy!