Line a 4 1/2 by 8 1/2 in. loaf pan with parchment, leaving overhang on 2 sides.
Heat 1 tbsp olive oil in large saucepan on medium. Add onion, half of the garlic, and 1/4 tsp salt and cook, stirring occasionally, until tender, 5 min. Add 2 cups water and bring to a boil. While whisking, slowly stream in chickpea flour and whisk vigorously, off of heat, until mostly lump free.
Transfer mixture to food processor with lemon zest and puree, gradually adding 1 tbsp oil until completely smooth. immediately transfer to prepared pan and smooth top. Cover with another piece of parchment and another loaf pan and press with a heavy object. Refrigerate until firm, 30 to 60 minutes.
Cut chickpea mixture into 1/2 inch cubes. Heat 1 tsp oil in small skillet and cook in 2 to 3 batches, turning occasionally, until browned, 3 to 5 minutes. Transfer to paper towel to drain.
Heat grill to medium high. Slice eggplants lengthwise, 1/2 inches thick. Brush eggplant slices with 1 tbsp olive oil, season with salt and grill until tender and lightly charred, about 3 minutes per side.
in a small bowl, whisk together yogurt, lemon juice, remaining garlic, and a pinch of salt. Drizzle yogurt sauce over eggplant and sprinkle with chickpea croutons and scallions.
Eggplant is porous, soaking up oil like a sponge as it cooks. To avoid sogginess, brush olive oil on slices (instead of drizzling) before grilling them.