Preheat the oven to 350°F (175°C).
Squeeze the drained sauerkraut well to remove as much liquid as possible. After squeezing all the water out, you should get about 1 cup of sauerkraut.
In a medium bowl, mix together Thousand Island dressing, cream cheese and Dijon mustard.
In 8×8-in (20x20cm) baking dish, combine corned beef, sauerkraut, Swiss cheese, reserving about ¼ cup for topping, and the sauce. Spread the mixture evenly.
Sprinkle reserved Swiss cheese and then mozzarella cheese on top.
Bake for 25-30 minutes, or until the dip is warmed through.
Cool slightly and serve warm with rye bread slices, or rye crisps.