In a small bowl, sift together the flour and baking powder. Reserve.
In a separate bowl, whisk together the egg yolks, buttermilk, and 3 tablespoons melted butter. Reserve.
In a medium skillet, heat 2 tablespoons of the oil. Over medium-high heat, saute the onions until golden, 6 to 8 minutes. Transfer to a large bowl, cool slightly, and stir in the dill. Stir in the flour mixture, salt, and then the egg yolk mixture.
Using a wire whisk or an electric mixer, whisk the egg whites until shiny and firm but not dry. Stir a little into the batter, and then fold in the remaining whites.
In a small bowl, whisk together the sour cream and dill and season with salt and pepper to taste. Refrigerate until needed. *
In a small bowl, combine the remaining 3 tablespoons melted butter with the remaining 3 tablespoons oil. Heat a 10-inch nonstick skillet or griddle and brush with some of the butter mixture. For each pancake, pour ½ cup of batter onto the griddle and, over a medium flame, cook until brown on one side. Turn and brown the other side, brushing the skillet or griddle with the butter mixture as necessary. (This makes a pancake approximately 6 inches in diameter). As the pancakes are cooked, transfer to a tray lined with a clean towel and keep warm in a low oven while preparing the remaining pancakes.
Presentation: Place one pancake on each of 8 or 10 heated plates. Spread some of the sour cream mixture over the pancake and arrange a slice of smoked fish on the cream. Squeeze a little lemon juice over the fish and garnish with a sprig of dill. Serve immediately.
To prepare ahead: Through step 4, keeping the batter chilled in a bowl of ice water.
Note: You can also make smaller pancakes, using 2 tablespoons of batter, making 2-1/2-inch pancakes. You can cut the smoked fish into smaller pieces or chop and stir into the cream as above.
*Salmon can be chopped, stirred into the sour cream, and spread over the pancake as above.
These look really delicious. You can watch a video where Wolfgang Puck demonstrates how to make them here.