Prawn and saffron risotto

Name: Prawn and saffron risotto
Prep time: 10 min
Cook time: 30 min
Total time: 40 min
Cuisine: European, Italian, Mediterranean
Ingredients: 560 g water, lukewarm; ½ tsp saffron threads; 4 spring onions/shallots, trimmed and cut into quarters; 2 garlic cloves; 2 tbsp olive oil; 180 g Arborio rice; 80 g dry sherry; 2 tsp vegetable stock paste (see Tips); 1 pinch sea salt, to taste; 1 pinch ground black pepper, to taste, plus extra to serve; 200 g raw prawns, shelled and cleaned; ½ lemon, zest finely grated, no white pith; 2 - 3 tsp butter (optional)