Gently melt the butter with the garlic, chopped parsley and salt in a saucepan over a low heat; don’t let the butter boil. Once melted, stir to combine, then cool. After 5 minutes, transfer to a small container and chill, but don’t let the butter solidify completely. Remove it from the fridge when it reaches the consistency of mayonnaise (after 15-20 minutes’ chilling) and stir well before use
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For the Marie Rose sauce, combine the mayo, ketchup and cayenne in a small bowl. Add Tabasco, lemon juice and seasoning to taste.
Preheat the grill to high. To prepare the langoustines, one by one lay them on their back and then cut them in half lengthways using a very sharp chef’s knife or good quality kitchen scissors. Remove any obvious intestinal tract.
Lay the halves, flesh-side up, on a large foil-lined tray. Spoon the soft garlic butter all over the tail meat, and then grill for 2-3 minutes. The tail meat is cooked just as it turns from translucent to white and the meat should be firm. Squeeze over the lemon juice.
Serve warm on a platter, spooning over the garlic butter from the grill tray, with the Marie Rose sauce on the side.
