For the pumpernickel breadcrumbs: Preheat the oven to 375 degrees F. Pulse the bread in a food processor several times to produce fine crumbs. Transfer the breadcrumbs to a baking sheet, pour the melted butter on top and toss to coat. Season liberally with salt and pepper, and then bake until lightly toasted and crispy, 12 to 15 minutes. Set aside to cool.
For the hollandaise and asparagus: Bring about 1 inch water to a boil over medium heat. Beat the sauerkraut juice and egg yolks in a medium heatproof bowl until foamy and light, about 30 seconds, and then set the bowl over the water, making sure it doesn't touch the water, and slowly whisk in the butter 1 tablespoon at a time. Continue whisking over the heat until all of the butter has been added and the hollandaise is thick and creamy, 8 to 10 minutes total. Remove from the heat, season with salt and pepper, and cover with plastic wrap and set aside.
Increase the oven temperature to 450 degrees F. Place the asparagus on a baking sheet, add the vegetable oil and toss to coat. Sprinkle liberally with salt and pepper, and then roast until the asparagus are tender and starting to brown on the edges, 14 to 16 minutes.
To serve, spoon the sauerkraut hollandaise over the asparagus and top with the pumpernickel breadcrumbs.