How to Make It
In a small bowl, mix the yogurt with the horseradish, capers and half of the lemon zest.
In a medium skillet, heat 2 tablespoons of the olive oil. Add the celery and onion and cook over moderately low heat until translucent, about 7 minutes. Transfer to a bowl and let cool. Add the salmon, bread crumbs, dill, egg, salt, pepper and the remaining lemon zest; mix gently and pat into 4 burgers.
In a large skillet, heat the remaining 3 tablespoons of olive oil until shimmering. Add the burgers and cook over moderately high heat until browned outside and medium in the center, about 4 minutes per side.
Spread the horseradish-dill sauce on both sides of the rolls. Add the burgers, close the sandwiches and serve.