Pre-heat oven to 375° F.
Trim away the leaves at the bottom of the cauliflower head and cut off the stem and tough core, keeping the head intact. Rinse and pat dry. Place cauliflower (florets-side up) in a cast-iron skillet or oven-proof pan. Drizzle with olive oil and season with salt and pepper. Cover pan with foil and cook for 30 minutes. Remove the foil, and continue to roast for an additional 30-40 minutes until the cauliflower is fork-tender and golden brown.
Meanwhile, spread hazelnuts on a sheet pan lined with parchment paper and place in the oven with the cauliflower. Roast for 6-7 minutes, making sure nuts do not burn. Remove from oven and slightly cool. Smash the toasted hazelnuts or give them a rough chop. Set aside.
Combine all the ingredients for the basil chimi in a blender, except the olive oil. Blend for 20 seconds to break down the leaves, shallot, and garlic. Slowly add olive oil in the opening of the lid in a steady stream while the machine is running, until well incorporated and slightly thickened. Taste for seasoning. Set aside or refrigferate if not using right away.
Transfer the roasted cauliflower to a platter and serve with basil chimi and toasted hazelnuts. Enjoy!