In a medium bowl, combine mayonnaise, sour cream, ketchup, relish, white onion, jarred horseradish, fresh lemon juice, Worcestershire sauce, yellow mustard, and kosher salt. Stir well, then refrigerate until needed.
Preheat an oven. Place sliced corned beef on a large oversized sheet of aluminum foil, then fold into a packet. Before sealing, add water, then close tightly. Place on a sheet pan then warm in the oven. Carefully open, as steam may escape!
Spread a tablespoon of softened butter on each slice of bread, then flip them over and spread Russian dressing on the other side.
Working with slices, top with warmed corned beef, sauerkraut, and overlapping slices of Swiss cheese. Place the remaining slices of bread (Russian dressing sides down) on top. Press down slightly.
Heat a large skillet over medium-low heat. Once hot, add sandwiches. Cover with a lid then cook. Carefully flip each sandwich over, then cook covered. Make sure the heat is not too high or you’ll burn the bread!
For that classic deli style presentation, cut each sandwich in half (diagonal or lengthwise is a personal preference!), then stack on top of each other and spear with small dill pickles on a bamboo skewer. Serve with your favorite potato chips and enjoy hot!