1. Put the prawns in a large baking dish.
2. Combine the oil, garlic, red chilli flakes and fennel seed in a small food processor and process until the garlic is somewhat paste-like. Pour this mixture over the shrimp and let them marinate for 30 minutes.
3. Light a grill to high heat.
4. Strain the prawns from the marinade and season them with salt and black pepper, to taste. Grill the prawns on both sides until they are slightly charred and just cooked through, about 2 minutes per side. (Chef's note: They can be left slightly undercooked since they will cook longer in the butter sauce.) Transfer the prawns to a plate.
5. Melt the butter in a large saute pan set over the grates of the grill. Add the lemon zest, lemon juice, tarragon and thyme leaves and cook for 30 seconds. Add the prawns and cook, tossing once, about 1 minute.
6. Finish with the parsley and adjust the seasonings, if necessary. Transfer the prawns to a serving platter and garnish with thyme and parsley.