Place water, onion, parsley, thyme, bay leaf, and peppercorns in a medium pan set over high heat; bring to a boil. Reduce heat to medium-low, add salmon, cover, and allow to simmer until the salmon is cooked through, about 8 minutes. Remove salmon from water; when cool enough to handle, flake salmon into small pieces, and transfer to a medium bowl. Add horseradish, sour cream, and salt; stir to combine. Set mixture aside.
Fill a large bowl with ice and water; set aside. Bring a wide, shallow pan of water to a simmer over medium heat. Add asparagus; let simmer until tender, about 3 minutes. Transfer to the ice bath; let cool. Transfer to a cutting board, and slice the asparagus in half lengthwise; set aside.
Assemble the sandwiches: Spread reserved horseradish-salmon mixture onto each slice of bread. Top half the slices with six asparagus-spear halves, then cover with the remaining slices of bread. Trim crusts, cut each sandwich into four triangles, and serve immediately.