How to Make It
In a medium bowl, whisk the buckwheat and all-purpose flour with 1 teaspoon of salt. Add the water and egg and whisk until smooth. Cover the batter and let stand at room temperature for 30 minutes.
Preheat the oven to 400°. Heat a 10-inch cast-iron skillet over moderate heat until hot, about 4 minutes. Alternatively, heat a 10-inch nonstick skillet. Soak a paper towel with vegetable oil and carefully rub a thin film of oil in the skillet. Pour in 1/3 cup of the crêpe batter and swirl the skillet to distribute it evenly over the bottom. Cook over moderately high heat until the crêpe is lightly browned on the bottom and the edge begins to curl, about 2 minutes. With a spatula, carefully flip the crêpe and cook on the second side for about 1 minute. Transfer the crêpe to a baking sheet. Repeat with the remaining batter to make 3 more crêpes.
Working on the baking sheet, fold the crêpes in half and then in half again to make triangles. Tuck the meat and cheese slices into the folds and bake the crêpes for about 4 minutes, until the cheese is melted. Open the crêpes and tuck a handful of arugula leaves in the center of each one. Drizzle with olive oil and season with salt and pepper. Close the crêpes and serve right away.