Make the Sauerkraut: Heat the oil in a large deep skillet over medium heat. Add the cabbage, onions and salt and sauté for 5 minutes, until slightly wilted. Stir in the mustard seeds, then add the cider vinegar, cider, and beer and bring to a simmer over medium-high heat. Adjust the heat to keep at a simmer and cook until the cabbage is fairly tender, but not mushy, about 15 to 20 minutes. Let the mixture cool to room temperature. Extra sauerkraut can be kept in its liquid in a tightly sealed container in the fridge for up to a week. (Make some hot dogs or sausages later in the week to use up the leftover sauerkraut!)
While the cabbage is cooking, make the Russian Dressing. In a small bowl combine the mayonnaise, ketchup, onion, relish, lemon juice and salt and pepper.
Spread one side of each of the slices of bread with the softened butter. Place 4 bread slices on a cutting board, buttered side down. Spread the slices with the Russian Dressing, about 1 tablespoon on each. Spoon about 2 to 3 tablespoons of well-drained sauerkraut onto each piece. Top with two slices of turkey, then 1/4 of the gruyere cheese. Place the remaining bread on top to finish the sandwiches, making sure the buttered side faces up.
Heat a large heavy skillet over medium heat. Add the sandwiches (you will probably have to cook them two at a time), and cook for about 3 minutes on each side, until the outside is nicely browned, and the cheese has melted.