Preparation Preheat oven to 425°F. Slice 1 cabbage into 1"-thick steaks. Cut the second cabbage in half (if using). Arrange on a large rimmed baking sheet and drizzle with oil. Turn steaks and halves and drizzle second sides with oil; season with 1 tsp. salt and 1/2 tsp. pepper. Arrange cabbage halves cut side down and roast until golden brown on the bottom and tender, 35–40 minutes. (Reserve cabbage halves for another use. Let cool, then cover and chill.) Meanwhile, place chickpeas on another rimmed baking sheet and pat dry with a clean towel. Add cumin, 1/4 cup oil, 1 tsp. coriander, 1/2 tsp. salt, and 1/2 tsp. pepper and toss to combine. Roast, stirring once halfway through, until golden and crisp, 20–25 minutes. Transfer to a medium bowl and set aside; reserve baking sheet (no need to wash). Tear bread into bite-size chunks. Whisk 1/4 cup oil, 1 grated garlic clove, 1 tsp. mustard, 2 Tbsp. chopped parsley, 1/2 tsp. salt, 1/4 tsp. pepper, and remaining 1/4 tsp. coriander in a large bowl to combine. Add torn bread and toss well to coat. As soon as chickpeas are done roasting, transfer bread to reserved baking sheet and roast until golden and crispy, 10–12 minutes. Whisk mayonnaise, lemon juice, and remaining grated garlic, 1 Tbsp. oil, 1 tsp. mustard, and 1/2 tsp. pepper in a small bowl. Taste and season with salt if needed. Smear mayonnaise sauce on plates and top with cabbage steaks. Top cabbage with chickpeas and croutons. Drizzle with more mayonnaise sauce, thinning with lemon juice or water if needed to make it pourable. Top with parsley leaves.