Preheat oven to 450F. On a rimmed baking sheet, toss cauliflower with 1/2 of the oil. Season with salt and pepper. Roast for 15-20 minutes. In a small bowl, combine remaining oil, panko, cheese, and rosemary.
Remove sheet from oven. Add beans, lemon zest, and red-pepper flakes to cauliflower, sprinkle with panko mixture and roast until panko is golden brown, about 5-7 minutes.