Preheat oven to 400°F.
Bring a large pot of salted water to a boil. Add the whole cauliflower and boil for 5 minutes. Drain and pat dry.
In a small bowl, mix together the olive oil, 1 tablespoon lemon juice, Dijon mustard, salt, and pepper. Rub this mixture all over the cauliflower.
Place the cauliflower on a baking sheet and roast for 30-40 minutes, until tender and golden brown.
In a small bowl, whisk together the tahini, water, 2 tablespoons lemon juice, and garlic.
In another small bowl, mix together the parsley, cilantro, and mint.
When the cauliflower is done, transfer to a serving plate. Drizzle the tahini sauce over the top and sprinkle the herb salsa over the cauliflower.
