Cauliflower Shawarma

Name: Cauliflower Shawarma
Ingredients: 1 medium cauliflower, leaves still attached; 3 tbsp olive oil; 3 generous tbsp spice mix (see below); 1 dried chili; 1 tbsp plus 1/2 tsp coriander seeds; 1 tbsp plus 1 3/4 tsp cumin seeds; 2 1/2 tsp ground allspice; 1 1/4 tsp white pepper; 1/2 tsp ground turmeric; 1 heaped tsp sea salt; 2 large onions, peeled and sliced; 1 tbsp olive oil; 1/2 tsp salt; 1 tsp sugar; 1 lemon; Scant cup tahini paste; 1/2 cup water; 3 tbsp roasted pine nuts; 1 tsp sumac (Optional); Crispy pita