Crêpes with Smoked Trout and Caviar

Name: Crêpes with Smoked Trout and Caviar
Ingredients: 2/3 cup all-purpose flour; Kosher salt; 1/8 teaspoon sugar; 1 cup milk; 2 large eggs; 3/4 cup crème fraîche; 3 tablespoons drained prepared horseradish; 2 tablespoons chopped dill, plus 12 dill sprigs for garnish; Freshly ground pepper; 2 tablespoons unsalted butter; 1 small red onion, thinly sliced; 1 small Golden Delicious apple, halved and thinly sliced; 1 smoked trout fillet (7 ounces), skinned, flesh flaked into 1/2-inch pieces; 2 tablespoons American caviar