Heat a skillet/pan large enough to fit your sandwich on the stovetop over medium heat.
Butter one slice of bread and place it butter side down into the pan.
Add a thin layer of mayo (if using) to the other side of the bread.
Add a 1/4 lb of sliced corned beef to the bread.
Then add 2 slices of swiss cheese to cover the meat.
Add the sauerkraut (as much as you'd like) on top of the cheese. Sprinkle salt and pepper on top.
Add another thin layer of mayo (if using) to the other slice of bread and put on top of the sandwich.
Butter the top of the bread before you flip the sandwich. Carefully flip the sandwich with a large spatula when it's ready, and cook the other side for a few minutes.
When it's done, carefully move it to a serving plate, cut it in half, and serve immediately with thousand island dressing on the side for dipping.
Repeat steps to make the next sandwich if you plan on making them all at once, or save the rest to make for later.
Notes
This recipe makes 4 large Reuben Sandwiches. You can absolutely use less meat if that is your preference.