Heat the oven to 425°F. Trim the outer leaves from the cauliflower and cut the florets from the core. It’s okay if they’re not all perfectly separated, but you want them about 1 inch across, so cut through some if necessary.
Put the garlic on the cutting board and hold a chef’s knife above them, parallel. Press down lightly to crush the garlic a bit; remove the peel and trim the flat end.
Put the cauliflower in a large gratin dish; drizzle with the olive oil and sprinkle with salt and pepper. Toss to coat the cauliflower with the oil and spread it out in an even layer. Nestle the garlic among the cauliflower florets and transfer the dish to the oven; cook until the florets are beginning to turn brown and the stems are crisp-tender (you’ll be able to pierce them with a thin-bladed knife, but they’ll offer some resistance), 10 to 15 minutes.
Remove the dish from the oven and remove and discard the garlic. Sprinkle the blue cheese evenly on top of the cauliflower, then sprinkle with the bread crumbs. Put the dish back in the oven and cook until the cheese is bubbling and the bread crumbs are golden brown, another 10 to 15 minutes. Serve hot or warm.