Roughly chop up turmeric, ginger, galangal, lemongrass, garlic, shallots and chilies then pop in a blender or liquidiser and process to a smooth paste using as little water as possible. Don't use a food chopper - the result will be much too coarse. If you prefer, you could pound everything in a pestle and mortar.
Heat 1/2 cup OR 6 tbs vegetable oil in a wok, deep pan or pot. When oil begins to smoke, put in spice paste and keep stirring over medium heat till paste is fragrant and you start coughing or sneezing. This should take about 5-8 minutes of stirring and frying.
Dump in mussels and mix well. Pour in coconut milk and water OR stock and bring to the boil. Add makrut and turmeric leaves, put lid on and lower heat. Simmer gently till mussels open. This should take 3 - 5 minutes, tops. Do not overcook (tough, rubbery mussels *Yuck*).
Season to taste, turn off heat and serve immediately with white rice or over scalded rice noodles. I like a sprinkling of coriander leaves and a squeeze of lime over it.