In a large bowl, whisk flour with salt. Whisk in eggs. Slowly add 2 cups water in a continuous stream, whisking continually until you have a thin batter. Chill in refrigerator for 15 minutes.
Lay out paper towels on a work surface. Heat 1 tablespoon oil in a 6-inch sauté pan over medium heat. Do not use too much oil, just enough to make sure the pan is coated. Pour batter in the center of the pan and, tilting the pan, swirl the batter to the edges. As soon as the bottom is cooked, about 45 seconds, transfer crepe to paper towels by sliding it out with a spatula or inverting the pan. You can stack the crepes if you separate them with waxed paper, otherwise they will stick together. Repeat until all batter is used.
Spread a layer of sour cream on one side of each of the crepes. Lay one slice smoked salmon on the sour cream. Sprinkle with onions. Roll up and place on a platter, seam-side down. Repeat until all crepes and filling ingredients are used.
Sprinkle with capers and chopped chives.