In a bowl, whisk the eggs with 1/2 cup of the milk and the salt. Whisk in the flour until smooth, then whisk in the remaining milk and the butter; the batter should have the consistency of cream. Let the batter rest for 1 hour.
Heat a 7-inch crêpe pan and brush with butter. Pour 1/4 cup of batter into the pan and rotate it immediately to coat the bottom evenly; pour any excess batter back into the bowl. Cook the crêpe over moderate heat until lightly browned on the bottom, about 30 seconds. Flip the crêpe and cook the second side until brown dots appear, about 10 seconds longer. Transfer to a large plate. Repeat with the melted butter and remaining batter to make 18 crêpes.
In a saucepan, melt the apricot jam with the lemon juice; stir until smooth.
Preheat the oven to 400°. Butter a 9-by-13-inch baking dish. Spread each crêpe with 1 teaspoon of jam and 1 tablespoon of walnuts. Fold it into quarters and transfer to the baking dish. Repeat with the remaining crêpes. Cover the dish with foil and bake for 10 minutes, or until hot. Sprinkle with the remaining walnuts and sift confectioners' sugar on top. Serve the crêpes warm, with chocolate sauce if you like.