Heat oil in a large saucepan over medium-high heat. Add spring onion and cook for 2-3 minutes until softened. Add coriander and sugar. Cook for 1 minute. Stir in laksa paste and cook for 3 minutes. Add coconut milk, stock, and 3 cups of water. Bring to a simmer and cook for 10 minutes for flavours to develop.
Meanwhile, place noodles in a heatproof bowl and cover with boiling water. Set aside for 2 minutes. Add marinara mix to coconut broth, cover, and cook for 3 minutes or until just cooked. Drain noodles and divide between 4 deep serving bowls. Ladle laksa into bowls and garnish with mussels (if using), basil, and sliced chilli. Serve immediately with lime wedges.