Pre-heat a large frying pan, iron skillet or griddle pan on a medium heat (not too high).
Make a basic Thousand-island dressing by mixing the first measure of mayonnaise with the ketchup. Spread the four slices of bread generously with this dressing and place a slice of Swiss cheese on top of two of the bread slices.
Build the sandwich with 2-3 shaved/slices of corned beef, a generous serving of sauerkraut, and then add the last slice of cheese. Close the sandwich with the remaining bread slices, dressing side down, and spread mayonnaise evenly on the top of each.
Carefully place the sandwiches, mayonnaise side down, into the warmed pan and while cooking spread mayonnaise again over the exposed tops. The trick is to grill low and slow until golden brown, about 4-5 minutes. Gently flip the sandwich, lightly press down with a fish-slice, and grill the other side for a further 4-5 minutes until golden.
Serve, cut in half, with a dill pickle on the side.