How to Make It
In a medium bowl, mix the flour with 1/4 teaspoon of salt and the sugar. Whisk in the milk and eggs until the batter is smooth. Let the batter rest for 15 minutes.
In a small bowl, mix the crème fraîche with the horseradish and chopped dill and season with salt and pepper.
Strain the crêpe batter through a fine sieve set over a small bowl. Heat a 6-inch crêpe pan or nonstick skillet. Add 1/2 teaspoon of the butter. When the butter stops foaming, add 2 tablespoons of batter, tilting the pan to evenly coat the bottom. Cook over moderately high heat until the edge of the crêpe is lightly browned, about 1 minute. Using a spatula, flip the crêpe and cook until lightly colored on the second side, about 30 seconds. Transfer the crêpe to a plate. Repeat with the remaining butter and batter, adjusting the heat as necessary so that the crêpes do not burn. Stack the crêpes as they're done; you should have 12.
Spread each crêpe with 1 tablespoon of the horseradish crème fraîche and top with 3 red onion slices, 2 or 3 apple slices and 1 heaping tablespoon of trout. Roll up the crêpes and transfer to plates or a platter. Garnish each crêpe with 1/2 teaspoon of caviar and a dill sprig and serve.