Make the juniper salt: Gently crush 2 tablespoons salt and the juniper berries on a cutting board with a flat side of a large heavy knife or in a mortar and pestle until coarsely ground. (Juniper salt can be stored, tightly wrapped with plastic wrap, 2 days.)
Make the sorrel-red onion butter: Stir together butter, sorrel, onion, capers, mustard, and lemon zest. Season with salt and peppercorns. (Butter can be refrigerated up to 1 week; flavors will intensify.)
Slather bread with butter, top with some smoked fish, and sprinkle with juniper salt.