Heat a large skillet or frying pan over medium heat.
Spread the softened butter evenly over one side of each slice of rye bread.
Place one slice of bread in the skillet, buttered side down.
Spread 1 tablespoon of the Thousand Island dressing over the exposed side of the bread.
Top with the sauerkraut, Swiss or Gruyère cheese, and salt beef.
Spread the remaining tablespoon of Thousand Island dressing over the unbuttered side of the second slice of bread, then place it on top with the dressing side facing down.
Press the sandwich gently with a spatula and cook until the bottom is crisp and golden brown, about 3 to 4 minutes.
Carefully flip the sandwich, place a sandwich press or a heavy skillet on top, and cook until the second side is golden brown and the cheese has fully melted, about 3 to 4 minutes more.
Transfer to a cutting board, let rest for 1 minute, then slice and serve immediately.