Heat a large pot of water to boiling. Lightly salt the water, add the cauliflower and lemon zest, and cook until the caulflower is crisp-tender, 4 to 5 minutes. Drain. Refresh in cold water and drain again. Transfer to a large mixing bowl.
Heat, but do not boil, the olive oil in a small saucepan. Remove the pan from the heat, drop in the pepper flakes, and let steep until the oil is red