For the pastrami rub, in a bowl add mustard seed and powder, coriander seed and powder, garlic and onion powder paprika ground black pepper and whole peppercorns. Mix these spices together.
Cover the entire corn beef with yellow mustard for the binder and season with the homemade pastrami rub. Put it in.
Preheat the grill to 250 degrees and set for indirect heat. Smoke the seasoned corn beef for 3 hours or until it reaches 170 degrees internally.
Next place it on a wire rack in an aluminum pan. Add beef stock to the pan and continue cooking for another hour or until the meat is probe tender at 200 degrees internally.
For the Russian sauce, in a mixing bowl add mayo, freshly grated onion, horseradish, ketchup and sriracha for some heat. Mix the ingredients together and set aside.
For assembly, preheat the griddle to 350 degrees. Spread mayo on the outside of the rye bread slices and place one facing down on the flat top.
Next add some mustard, Swiss cheese, sliced smoked corn beef, spoonful of sauerkraut, Russian sauce, a little more Swiss cheese and close it with another slice of rye bread mayo side facing up. After 5 minutes, once the bottom bread slice has color and the cheese is starting to melt, flip the entire sandwich and cook for another few minutes.