Make the filling:
In a saucepan cook the onion in the butter over moderately low heat, stirring, until it is softened, add the flour, and cook the mixture, stirring, for 3 minutes. Add the milk in a stream, whisking, and cook the mixture over moderate heat, whisking, until it is thick. Stir in the nutmeg, the dill, the spinach, and salt and pepper to taste and let the filling cool.
Make 12 crepes (procedure follows) with the dill crepe batter. Spread 2 tablespoons of the filling on each crepe and roll the crepe up jelly-roll fashion. Arrange the crepes, seam sides down, in a buttered shallow baking dish just large enough to hold them in one layer. The crepes may be prepared up to this point 1 day in advance and kept covered and chilled. Brush the crepes lightly with the melted butter and bake them in the middle of a preheated 400F. oven for 20 minutes.
Make the sauce:
In a small saucepan bring the broth to a boil. In a bowl whisk together the eggs and the lemon juice. Add half the broth to the egg mixture in a stream, whisking, and whisk the mixture into the remaining broth. Heat the sauce, stirring, until it reaches 170F. on a candy thermometer and is thickened slightly, but do not let it boil, and add salt and pepper to taste.
Divide the crepes among plate and drizzle the sauce over them.
Make the dill crepe batter:
In a blender or food processor blend the flour, 1/2 cup plus 2 tablespoons water, the milk, the eggs, the butter, and the salt for 5 seconds. Turn off the motor, add the dill, and with a rubber spatula scrape down the sides of the container. Blend the batter for 20 seconds more, transfer it to a bowl, and let it stand, covered and chilled, for 1 hour. The batter may be made 1 day in advance and kept covered and chilled. Makes enough batter for about 13 crepes.
Make the crepes:
Heat a crepe pan or non-stick skillet measuring 6 to 7 inches across the bottom over moderate heat until it is hot. Brush the pan lightly with the butter, heat it until it is hot but not smoking, and remove it from the heat. Stir the batter, half fill a 1/4-cup measure with it, and pour the batter into the pan. Tilt and rotate the pan quickly to cover the bottom with a layer of batter and return any excess batter to bowl. Return the pan to the heat, loosen the edge of the crepe with a spatula, and cook the crepe for 1 minute, or until the top appears almost dry. Turn the crepe, cook the other side lightly, and transfer the crepe to a plate. Make crepe with the remaining batter in the same manner, brushing the pan lightly with butter as necessary. The crepes may be made 3 days in advance, kept stacked, wrapped in plastic wrap, and chilled.