Sauteed potatoes topped with caviar and plain yogurt make a rich filling for buttermilk crepes.
In a medium bowl, combine flour with 1/8 teaspoon salt. In small bowl whisk together egg, egg whites, and buttermilk. Slowly whisk egg mixture into flour until batter has consistency of heavy cream.
Heat oven to 200 degrees. Meanwhile, in a large nonstick skillet, heat oil over medium-high heat. Add potatoes, remaining 3/4 teaspoon salt, and pepper; cook, tossing occasionally, until potatoes are golden brown, about 12 minutes. Add stock; cook, tossing, until all liquid is absorbed, about 1 minute. Remove from heat; cover.
Meanwhile, place medium nonstick skillet over medium-high heat; lightly spray with cooking spray. Pour 3 tablespoons batter into center of skillet; swirl pan to create crepe about 5 inches in diameter. Cook until speckled golden brown, 2 to 3 minutes per side. Place in oven to keep warm. Continue making crepes until all batter is used.
Place crepe on work surface; place a heaping tablespoon of potatoes in center. Top with tablespoon of yogurt and 1/4 teaspoon of each caviar. Add sprig of chervil; roll up. Repeat the process with remaining crepes and fillings.
Weather can affect the consistency of crepe batter; if it's too thick to swirl in the pan, add a bit more buttermilk.