Make chickpea patties: In a 3- to 4-qt. pan, mix chickpea flour and dry seasonings. Gradually whisk in 2 1/2 cups water until smooth. Cook over medium heat, whisking often, until mixture begins to thicken, 3 to 5 minutes. Switch to a spoon and stir until mixture pulls away from pan bottom and mounds in center of pan, 4 to 6 minutes. Stir in parsley. Using an oiled spoon or fingers, evenly spread mixture in an oiled 9- by 13-in. pan. Let cool.
Make mayo: Mix all ingredients together in a small bowl. Chill, covered.
Make slaw: Whisk together vinegar, oil, salt, and pepper in a medium bowl, then gently stir in carrots and radishes.
Now for the sandwiches! Preheat broiler and set a rack 4 in. from heat. Split rolls, leaving one side attached, and open up. Place rolls cut side up in a large rimmed baking pan; lightly brush tops with 1 tbsp. olive oil. Broil until golden, about 1 minute. Tip onto counter and drape with foil.
Oil the baking pan. Brush chickpea mixture with 1 tbsp. olive oil; sprinkle with salt. Cut into 12 rectangles and set slightly apart in oiled pan. Broil until golden, 4 to 6 minutes. Turn patties over and broil until golden on second side, about 2 minutes.
To assemble, generously spread mayo on cut sides of rolls. Place some salad greens on one side of each roll and top with some carrot slaw, then 2 chickpea patties.
*Find in the baking aisle of well-stocked grocery stores and at health food stores.
Make ahead: Wrap and chill uncooked chickpea mixture and mayo up to 1 day.
Sorry but this was gross. I used gram flour which is the Indian version of chickpea flour and what I use for pakoras and have used in other breading applications, maybe that was the issue? The patties had a weird bitter aftertaste that did not work. Not wanting to waste too much food we ended up eating this sans patties - the mayo and carrots were fine but the sandwich was slim without the patties. I ended up frying up some eggs to put on the sandwiches (just so there was something else) and pulling out a bag of frozen mixed veggies just to bulk out our meal. I liked the idea though, I may try subbing in some cooled, cut and broiled polenta for the chickpea patties if I tried this again.