For the red curry paste, combine the onion, lemongrass, garlic, ginger root, red chili, kaffir lime leaves, cilantro, turmeric, tomato paste, lime zest, sunflower oil, and a pinch of salt in a food processor. Blend until it forms a coarse curry paste.
Check if all the mussels are intact. Discard any broken ones. Take a wok or large pot and add 2 tablespoons of curry paste. Cook for 1 minute, then add the coconut milk. Whisk everything together well and bring it to a boil.
Once the liquid is boiling, add the mussels and mix them well with the sauce. Cover the pot and cook the mussels for 5-7 minutes. Stir the mussels occasionally. When all the mussels have opened, add half of the spring onions, red chili, and coriander, and mix everything well.
Serve the mussels and garnish with the remaining spring onions, red chili, coriander, lemongrass, and lime.