Place a rack in the bottom of the oven and preheat to 230°C (450°F). Bring a large pot of salted water to a boil. Trim the cauliflower stalk flush with the head, then pierce the stalk twice with a paring knife to make a plus sign. Boil the cauliflower stem-side down, covered, until a knife slides easily into the core, 6 to 8 minutes. Drain and cool for 10 minutes. Put it on a baking sheet, rub with 2 tbsp olive oil and roast until golden and tender, about 50 minutes, rotating halfway. Brush with 3 tbsp more oil and broil on high until well caramelized, about 10 minutes. Keep warm. Heat the remaining 2 tbsp oil with the anchovies in a saucepan over medium heat, crushing them until dissolved. Add capers and garlic and cook until fragrant, 2 minutes. Crush the tomatoes by hand into the pot, keeping the juice back. Simmer until they break down, 4 minutes, then add the juice and reduce slightly, 3 minutes. Smash, halve and pit the olives, add to the sauce and cook 1 minute. Off the heat, stir in the basil and season with salt. Spoon the sauce onto a dish, set the cauliflower on top and drizzle with oil.