To make the laksa paste, combine the onion, chillies, ginger or galangal, garlic, lemongrass, shrimp paste, if using, coriander stems, ground coriander, cumin, paprika, turmeric and ¼ cup (60ml) water in a blender. Blend until smooth and well combined.
Heat a large saucepan over medium-high heat. Add the oil and laksa paste. Cook, stirring, for 2 mins or until aromatic. Add the stock and coconut cream and bring to the boil. Reduce heat to medium. Simmer for 5 mins or until the mixture thickens slightly. Add the sugar and fish sauce and cook for 5 mins.
Add the chicken, prawns and tofu to the coconut cream mixture and stir to combine. Cook for 5 mins or until chicken is cooked through and the prawns curl and change colour. Remove from heat. Squeeze over 2 lime halves. (You can add extra lime juice, fish sauce or sugar, if needed.)
Divide the noodles among serving bowls. Top with bean sprouts. Ladle over the soup and top with chicken, prawns and tofu. Serve with coriander leaves, fried shallots, chilli sauce and remaining lime.