Preheat oven to 500F. Line a baking sheet with foil.
Combine 10 cups water, 2 tbsp oil, 2 garlic cloves, 2 sprigs rosemary and 1 tbsp salt to a boil. Add cauliflower, cover, and reduce heat to medium. Cook until tender, 12 to 15 minutes. Remove to a cooling rack. Let stand 10 minutes.
Meanwhile, mince remaining 2 garlic cloves. Finely chop 1 tsp rosemary. Transfer both to a small bowl. Stir in tahini, dijon, poultry spice, paprika, pepper, remaining 1 tbsp olive oil, 1/2 tsp salt and 5 tsp water. Brush over cauliflower.
Bake until cauliflower is golden-brown and crust has set, about 20 minutes. Garnish with parsley and serve immediately.