Preheat the oven to 200° C. Place portobello slices in a large bowl and drizzle over balsamic vinegar, olive oil, liquid smoke and a good pinch of smoked sea salt and pepper. Toss to coat. Place in one layer on a lined baking tray. Bake for 25 minutes or until golden and caramelised. Allow to cool slightly.
In a small bowl, whisk together the mayonnaise with the tomato paste. Season with salt and pepper to taste. Set aside.
Preheat the grill. Arrange the bread on a large baking sheet and brush with olive oil. Grill until lightly toasted, 1 to 2 minutes. Transfer 4 slices of the bread to your work surface. Flip the remaining 4 slices on the baking sheet and top with the dressing, mushroom slices, cheese and sauerkraut. Return to the grill until the cheese is slightly melted. Close the sandwiches, cut in half and serve.
For mayonnaise: Place first 5 ingredients in a food processor, pulse 3 - 4 times till combined. Very slowly drizzle in oil till ingredients start to emulsify. Makes 1 cup. Store in refrigerator for up to 1 week in an airtight.