For this classic Scandinavian sandwich, thin, even slices of hard-boiled egg are used.
Fill a medium stockpot with about 1 inch water. Squeeze half a lemon into water; add the lemon rind and a few dill sprigs. Bring to a boil. Put a steamer insert into pot. Add shrimp, and season with salt. Cover, and steam until cooked through, about 3 minutes. Transfer the shrimp to a plate; let cool completely. Discard dill and lemon.
Make the dressing: Stir together mayonnaise, creme fraiche, 1/4 teaspoon salt, chopped dill, and 1 teaspoon lemon juice (from remaining lemon half) in a bowl.
Cut shrimp into 1/2-inch pieces; transfer to a small bowl. Add 1/4 cup dressing, and toss to combine.
Spread bread slices with dressing, and top with egg slices. Cut each sandwich in half on the diagonal. Spoon about 1 tablespoon shrimp salad onto each half. Garnish with dill sprigs.