Position a rack in the middle of the oven and preheat to 400°F. Line a baking sheet with parchment paper.
Peel off most of the papery outer layer from the head of garlic, but leave the head intact. Cut across the very top of the head to expose the cloves. Place the garlic on a sheet of foil and drizzle with 1 tablespoon olive oil. Crumple the edges of the foil around the garlic to make a packet for roasting, and set aside.
In a medium bowl, combine the cauliflower, salt, and 2 tablespoons of the olive oil and toss until the cauliflower is well-coated. Spread the cauliflower on the lined baking sheet without overcrowding, and nestle the garlic next to the cauliflower. Roast the vegetables on the middle rack for about 40 minutes — the garlic may take about 10 minutes longer than the cauliflower to roast, so check its doneness (it should be soft and spreadable) when you take the cauliflower out of the oven.
Transfer the roasted cauliflower to a food processor. Add the remaining 2 tablespoons olive oil, lemon juice, and pepper to taste. Squeeze the roasted garlic out of its skins into the processor and then pulse until completely smooth and spreadable, 3 to 4 minutes. (If it is still chunky after a few minutes of blending, try adding a tablespoon or two of water or a little more oil.)
Serve with plenty of extra lemon wedges (the lemon juice will turn the purple mixture hot pink), salt, and pepper.
To make AIP friendly omit: Pepper