Preheat the oven to 425°F. Line a large baking sheet with parchment paper.In a large bowl, combine the hash browns, 2 tablespoons of the melted butter, the onion, ½ teaspoon of the salt, and the pepper. Spread the hash brown mixture into an even layer on the prepared baking sheet and bake for 20 minutes. Move the sheet directly under the broiler and switch the oven to broil. Broil for 1 to 2 minutes more, until the hash browns are lightly crisped and golden.Form the “biscuits” by filling a 4-inch biscuit cutter with one-quarter of the baked hash browns. Repeat with the remaining hash browns to make 4 “biscuits” total. Set aside.In a small bowl, combine the mayonnaise, relish, and tomato paste. Set aside.In a blender, combine the egg yolks, lemon juice, and hot sauce. With the blender running on low speed, very slowly drizzle in the remaining 8 tablespoons (½ cup) melted butter. Blend until the hollandaise sauce has thickened slightly, about 2 minutes. Set aside.To assemble, lay the slices in a medium skillet over medium-low heat. Add 2 tablespoons of water and cook until the pastrami is heated through, 1 to 2 minutes. Transfer the pastrami to a paper towel–lined plate.Layer each hash brown “biscuit” with 1 tablespoon of the Reuben sauce, a tomato slice, ¼ cup of the baby spinach, ¼ cup of the sauerkraut, one-quarter of the pastrami, and a poached egg. Drizzle with the hollandaise, sprinkle with the smoked paprika, and serve.
