In a medium bowl, whisk the flour with the salt and sugar. Whisk in the milk, followed by the eggs and then the butter. Refrigerate for 1 hour.
In a small bowl, blend the mayonnaise with the chopped dill, lemon zest and juice and season with salt and pepper.
Heat a 6-inch nonstick skillet (measured across the bottom). Lightly oil a paper towel and use it to grease the skillet. Add about 3 tablespoons of the crêpe batter to the skillet, swirl to coat the bottom of the pan and pour the excess batter back into the bowl. Cook the crêpe over moderately high heat until browned around the edges, about 1 minute. Flip the crêpe and cook for about 30 seconds longer; transfer the crêpe to a baking sheet. Repeat with the remaining crêpe batter, rubbing the skillet with oil as needed and reducing the heat to moderate if the skillet gets too hot. You should have about 18 crêpes, but you'll only need 16.
On a work surface, arrange 4 crêpes overlapping in a row that measures 19 inches. Arrange another row of 4 overlapping crêpes just below to form a neat 19-by-9-inch rectangle. Spread half of the smoked salmon slices over the lower row of crêpes. Starting at the bottom of the rectangle, roll up the crêpes neatly and tightly around the salmon. Trim the ends and slice the roll crosswise into 1-inch spirals. Repeat with the remaining crêpes and salmon. Transfer the hors d'oeuvres to a large platter.
Dot 1/2 teaspoon of the dill mayonnaise on each spiral, sprinkle with red onion and top with a dill sprig. Secure each spiral with a toothpick and serve.
The crêpes can be refrigerated for up to 6 hours before slicing.