Heat one side of your Blackstone griddle (or other large griddle) to medium-low.
Spread 1 teaspoon mayonnaise on the outside of each slice of bread. This will be the outside of your sandwich.
Spread 1 tablespoon Thousand Island dressing on the inside of each slice of bread. This will be the inside of your sandwich.
Top each slice of bread with one slice of Swiss cheese.
Divide the corned beef and sauerkraut evenly between two slices of bread, then top with the remaining slice of bread and cheese.
Cook, flipping once, until the bread is golden brown and the cheese is melted, 4 to 6 minutes per side. If possible, keep the sandwiches covered with the griddle lid while they are toasting.
Slice in half and serve hot.
Notes
Don't rush it--Don't be tempted to rush the process by turning up the heat. Low heat will gently toast the bread and allow the cheese to melt, while higher heat may cause the bread to burn before the cheese is melted.