Sauce:

Mix mustard, horseradish, and dill pickles. Set aside.

Crispy Onion Rings:

Line a baking sheet with several layers of paper towels. 

Prepare 1 bowl to fit inside another bowl that is filled with ice water (this will keep gluten from forming and will keep the batter crispy and light).

Whisk beer, flour, rice flour, baking powder, cayenne, salt, and pepper in smaller bowl to form the batter.

Heat oil in a heavy gauge pan to 360°F. Dredge onion rings in rice flour, a few at a time, then dredge in batter, and fry in hot oil until golden brown. Transfer to the prepared baking sheet and sprinkle with salt.

Assembly:

 Slice half-inch thick slices of rye bread or gather 8 to 12 slices of bread. Slather with mustard-horseradish sauce.

Layer with several tablespoons of sauerkraut. Add slices of hot pastrami and top pastrami with several crispy onion rings. Top with another dollop of horseradish-mustard and top with another slice of bread.

EAT!