Bring a large pot of water to a boil over high heat. Add the heads of cauliflower and par-boil until slightly tender, about 15 to 20 minutes.
Meanwhile, preheat the oven to 375ºF.
Carefully remove from the boiling water using tongs and place on a baking sheet lined with parchment paper or in a baking dish. Pat the cauliflowers very dry using paper towels.
Drizzle each head of cauliflower with olive oil and season with salt and pepper. Roast for 40 to 45 minutes until golden brown and very tender when pierced with the tip of a knife.
Meanwhile, make the green tahini sauce. In the carafe of a blender add the tahini, lemon juice, olive oil, preserved lemon, garlic cloves, fresh herbs, and jalapeno. Blend until smooth adding water as needed until a nice sauce forms. Season with salt and pepper to taste.
Drizzle the sauce all over the heads of cauliflower, sprinkle with pumpkin seeds and serve.
Optional, grill half of a lemon on a grill pan and squeeze over the cauliflower to garnish.