Simple Thai noodle soup

Name: Simple Thai noodle soup
Prep time: 30 min
Cook time: 30 min
Total time: 1 h
Cuisine: Asian, Thai
Ingredients: 1 good knob fresh root ginger, peeled and roughly chopped; 1-2 red chillies, seeds removed (optional), roughly chopped; 1-2 stalks lemongrass, tough outer layers removed, roughly chopped; 2-3 garlic cloves, peeled; 1 handful fresh coriander, a few sprigs reserved and chopped; a glug or two vegetable oil; 1 tbsp coriander seeds; 1-2 pinches ground turmeric; small jug vegetable stock; a couple of handfuls rice noodles; 1 x 400ml/14fl oz can coconut milk; 1 large handful raw, peeled tiger prawns (the shells and heads can be reserved for making stock in another recipe); a few splashes Thai fish sauce, to taste