Preheat oven and roast chickpeas: Preheat oven to 180°C fan forced (356°F). Place chickpeas on an oven tray and roast for 20-25 minutes, until dark and well toasted.
Process chickpeas: Set aside to cool briefly, then process to a powder in a food processor. Sieve powder through a fine sieve and measure out ½ cup (75g).
Prepare paste: Pound lemongrass stems in a mortar and pestle. Add ginger and garlic, continue to pound. Add 4 of the shallots and pound until mixture becomes a coarse paste.
Cook spices and paste: Heat oil in a large, deep frying pan over high heat. Add turmeric, fry for 30 seconds. Add paste, season with salt and cook for 1-2 minutes, stirring, until aromatic. Stir in remaining spices, pepper and shrimp paste, and cook, stirring, for 2 minutes.
Add liquids and simmer: Add fish sauce and continue to cook until the mixture “cracks.” Stir in the coconut milk and 2 cups (500ml) water. Bring to a simmer. Simmer for 5–10 minutes, allowing the flavours to develop.
Thicken curry: Mix the ½ cup chickpea flour with ⅔ cup (160ml) water in a small bowl to make a slurry. Add into curry and stir until well incorporated. Simmer until starting to thicken.
Cook prawns: Add the prawns and continue cooking for 2-3 minutes until they are just cooked through.
Prepare noodles and serve: Soak noodles in boiling water for 2 minutes, until soft. Drain and divide between serving bowls. Ladle over the curry, and garnish with thinly sliced shallots, lime wedges, mustard greens and coriander.