To make the sauerkraut, simply slice the cabbage in to thin ribbons after removing the core and place it in a large saucepan.
Add the apple cider vinegar, water, sea salt and caraway or fennel seeds and cook down over medium heat, stirring regularly, for 35 - 40 minutes. Add more water if the cabbage begins to brown at the bottom.
Remove from the heat.
To make the Russian dressing combine all ingredients in a bowl and stir to combine.
Finely slice the mushrooms. Heat a little oil in a large fry pan and sautée the garlic for 20 seconds before adding the mustard and fennel seeds.
Add the mushrooms and toss through the spices. Cook for four minutes before seasoning with sea salt.
To assemble the sandwich, lightly butter one side of the rye bread slices and place them butter side down on a pan. Grill until lightly toasted.
Remove from the pan and smear the unbuttered side of one slice with a good dollop of the dressing before following with a generous handful of the mushrooms. If you are using cheese slices, add a slice now then add the sauerkraut and slices of the dill pickle. If you are using the cheese sauce, add a drizzle now.
Place the second slice of bread on top (buttered side out) and return the sandwich to the pan and lightly grill until warmed through.
My quick and dirty sauerkraut can be stored in the fridge for a week.