Place the coriander seeds and peppercorns in a resealable freezer bag. Using a mallet or a rolling pin, crack the spices. Transfer to a bowl; mix in the kosher salt and sugar. Line an 8-inch square dish with plastic wrap, leaving a 3-inch overhang; place the salmon, skin side down, in the center. Cover with the spice mixture. Wrap tightly with the plastic wrap. Using a sharp knife, pierce the plastic several times on the top and bottom to drain. Place a plate on top of the fish, then a large, heavy can. Refrigerate 2 days, turning the fish once.
Bring a saucepan of water to a boil; season with salt. Set aside about 1/4 cup dill sprigs. Add the remaining dill to the pan; stir 5 seconds. Drain; transfer to a bowl of ice water to cool. Squeeze dry; transfer to a blender. Add the mayo; puree until the dill is finely chopped. Season with salt.
In a medium bowl, toss the cucumber, radishes and vinegar; season with salt.
Unwrap the fish. Rinse well; pat dry. Using a sharp knife, thinly slice on a sharp angle against the grain.
Divide pumpernickel among plates. Spread with the dill mayo. Top with the salmon; squeeze lemon over. Top with the cucumber and radish salad and the reserved dill sprigs.
