For the spiced butter sauce, place all of the ingredients in the bowl of a food processor and blitz. Set aside.
For the serving ingredients, heat the oil in a frying pan and when very hot, add the rice sticks and caramelise until the outside is crispy and the inside is still soft. Set aside.
For the Cajun spice mix, mix all of the ingredients in a bowl. For the shellfish cooking liquor, add 125ml/4fl oz water and the rest of the ingredients to a saucepan and bring to a very low simmer for 10 minutes. Add the mussels, cockles, clams and razor clams to the cooking liquor, cover and cook for 1–2 minutes and then add the langoustine for 2 minutes, or until the langoustine is cooked through.
In a separate saucepan, heat the spiced butter sauce until it is foaming and starts to turn golden.
Drain the shellfish, retaining the liquor. Pick out the garlic pieces and lemongrass and return the shellfish to the liquor, adding the crispy rice sticks and half of the coriander and mint.
Transfer the gumbo to warmed bowls and pour the spiced butter over the top. Squeeze over the lime juice and garnish with the remaining herbs and spring flowers. Serve immediately.