Mix all the ingredients for the Thousand Island dressing well. Set aside.
In a skillet, over medium heat, add the corned beef and heat up the meat until warmed through. Set aside. Heat up the sauerkraut until warm and set aside. Wipe out the pan so you can use it to cook your reuben.
Lay down both pieces of bread and layer the ingredients as follows: swiss cheese on both slices of bread. I like to trim the cheese so there's not much sticking out of the bread so it doesn't melt all over the pan.
Spread a generous amount of thousand island dressing on both slices of bread on top of the swiss cheese.
On one side, add the corned beef then top with sauerkraut.
Top the corned beef and sauerkraut side with the other slice of bread with cheese and dressing.
Over medium heat, warm up your pan and spray with cooking spray.
Thinly coat the outside of the sandwich bread that is facing up with mayonnaise...not thick, just lightly coat.
Place the sandwich with the mayonnaise side down in the pan and cook until golden brown 2-3 mins. As it's cooking, carefully, thinly coat the bread that is side up in the pan with mayonnaise.
Flip the sandwich and cover the pan, lowering the heat to medium-low. Cook until the cheese is melted and the bread is golden brown, about 2 minutes.
Serve hot.