Rub the bottom of a shallow baking dish with cold butter. Arrange salmon in dish. Season the fish with salt and pepper. Add broth and a few sprigs of dill to the pan. Roast the fish 12 to 15 minutes.
Combine sugar, salt and vinegar in the bottom of a medium bowl. Add chopped onions, chopped cucumber and radishes. Sprinkle in 3 tablespoons chopped dill and toss the mixture to coat in vinegar, sugar and salt.
Remove fish from the oven. Spoon pan juices over fish and carefully transfer to individual plates or platter. Top fish with relish and serve.