Make crepes:

Blend milk, flour, egg, and 2 teaspoons oil in a blender until smooth. Add chives and dill and pulse 1 or 2 times to just combine. Chill batter, covered, 30 minutes.

Stir batter to redistribute herbs. Lightly brush a 10-inch nonstick skillet with oil, then heat over moderately high heat until hot but not smoking. Holding skillet off heat, pour in half of batter (1/4 cup), immediately tilting and rotating skillet to coat bottom. (If batter sets before skillet is coated, reduce heat slightly for next crepe.) Return skillet to heat and cook until crepe is just set and pale golden around edges, 10 to 15 seconds. Loosen edge of crepe with a heatproof plastic spatula, then flip crepe over carefully with your fingertips. Cook until underside is set, about20 seconds more. Transfer crepe to a plate. Make another crepe in same manner, brushing skillet again with oil.

Prepare filling and assemble hors d'oeuvre:

Stir together cream cheese, lemon juice, zest, and pepper in a small bowl until smooth.

Put 1 crepe, browned side up, on a work surface, and spread with all of cheese mixture. Arrange salmon in an even layer over bottom half of crepe (side nearest you), then scatter radishes over salmon. Beginning at bottom, tightly roll up crepe, then cut roll crosswise into 4 pieces, trimming ends if desired.

Cooks' notes: Crepes can be made (but not filled) 1 day ahead and chilled, layered between sheets of wax paper and then wrapped in plastic wrap. Cheese mixture can be made 1 day ahead and chilled, covered. Bring to room temperature before using. Crepe can be rolled with filling (but not cut) 1 hour ahead and kept, wrapped in plastic wrap, at room temperature. Cut into pieces just before serving.