Make the dressing: in a small bowl, whisk mayonnaise, sriracha, horseradish, lemon juice, chopped onion and paprika. Set aside.
Spread 1/2 tablespoon of butter on one side of each slice of bread. Spread one tablespoon of the Russian dressing on the other side of the bread.
Place 4-6 slices of corned beef on the dressing, then 2-3 tablespoons of sauerkraut, then 1-2 slices of Swiss cheese. Top with slice of bread, sauce side down.
In a hot skillet, cook sandwiches over medium heat 5-6 minutes, flipping once. Cook until cheese melts and the bread is crispy.