Preheat your Traeger Flatrock griddle using two temperatures. One zone medium and one zone medium-high.
Assemble your ingredients.
Begin by placing your corned beef on the medium-high side of the griddle, heating briefly.
Spread butter or mayo on one side of each slice of bread, and place the bread spread side down on the griddle.
Put Thousand Island dressing on the side of the bread facing up.
Top the dressing with slices of cheese.
Then top the cheese with sauerkraut.
Once the meat is heated through, place meat on two of the pieces of bread.
Stack the bread to create a double-decker sandwich.
Move the sandwiches to the medium side of the griddle and cover with a steam dome.
Add a bit of water and allow the sandwiches to cook until golden on one side and the cheese begins to melt.
Flip the sandwich and cover again until melted and hot throughout.
Slice the Reuben sandwich in half and serve with extra Thousand Island dressing.