The grilled eggplant can be prepared ahead and refrigerated overnight. Just bring it to room temperature before using. Slideshow: More Gluten-Free Recipes
In a medium bowl, whisk the buckwheat flour with the eggs and half of the milk until lump-free. Add the remaining milk and whisk until smooth. Refrigerate the batter for 20 minutes.
Light a grill or preheat a grill pan. Brush each piece of eggplant on both sides with olive oil and season with salt. Grill over high heat, turning once, until soft, about 2 minutes per side.
In a small bowl, whisk the lemon juice with the mint, capers and the 2 tablespoons of olive oil.
Line a plate with wax paper. Heat an 8-inch nonstick skillet and lightly brush with olive oil. Add 1/4 cup of the batter to the skillet and swirl to coat the pan. Cook over moderate heat until the edges begin to pull away from the pan, 30 seconds to 1 minute. Flip the crêpe and cook 30 seconds to 1 minute longer. Transfer the crêpe to the prepared plate and repeat with the remaining batter.
Top the crêpes with the eggplant and the mâche. Drizzle with the lemon-herb sauce and serve.