Prepare an outdoor barbecue with a medium-high fire. Without removing the shells, slit about three-quarters of the way through the prawns down the ridged back and remove the vein that runs down the centre. Rinse and pat the prawns dry.
Whisk the thyme leaves, lime juice, one tablespoon of the olive oil, half a teaspoon of the salt and black pepper, to taste, in a shallow nonreactive bowl or dish. Lay the prawns cut side down in the lime mixture and refrigerate for 30 minutes.
In a food processor, pulse the garlic, jalapeno, spring onions, remaining one tablespoon of olive oil and remaining 1/2 teaspoon of salt to make a coarse paste. Add the coriander and pulse just enough to incorporate into the mixture. Spoon the paste into the slits in the prawns and close the openings.
Barbecue the prawns shell side down (to keep filling from falling out) for three minutes. Turn the prawns, cover, and barbecue another 2 minutes or until the prawns turn pink and are slightly firm to the touch. Sprinkle with salt and serve.
Cook's notes: Prawns cooked in the shells are more intensely flavourful. Leaving the shells on provides a buffer against overcooking, a misfortune many prawns suffer. Prawns, even these jumbos, continue to cook once removed from the barbecue; it's always best to cook the delicate shellfish until just opaque and let them finish cooking off the heat.
Jumbo prawns in the shell can be a knife and fork sort of deal, unless you're outside and it's summer and you are feeling very relaxed. Serve these with lots of napkins if your crowd is the peel-and-eat type.