Make the pickled onions: In a medium, microwave-safe bowl, combine the water, cider vinegar, sugar and mustard seed. Heat to boiling in the microwave (2 - 2 1/2 minutes). *Alternately, heat to a boil in a saucepan on the stovetop. Add the sliced red onion and season with salt and pepper. Stir and let stand at least 1 hour, before draining and using. (Refrigerate any leftovers to use for other things)
Make the dressing: Combine all the dressing ingredients in a small bowl. Stir to combine well. Cover and refrigerate until needed.
To prepare the sandwich: Lay out two of your bread slices. Spread with some of the Thousand Island Dressing. Top with cooked corned beef slices and two slices of Swiss cheese, overlapping them. Add a layer of pickled onions. Spread the inside of the two other slices of bread with Dijon mustard and place on top, mustard side down.
Heat a skillet over medium heat. Spread top of sandwiches with softened butter. Place into hot skillet, butter side down and grill until golden brown underneath. Before flipping, spread the top piece of bread with butter, then flip and grill that side until golden.
Be sure to read the Ingredient notes and FAQ for more tips on making this delicious sandwich.