Start by preparing the sauce; heat the olive oil in a large saucepan over a medium heat. Sweat the onion, garlic and celery for 6-8 minutes, stirring occasionally, until softened. Add the remaining ingredients and some seasoning and bring to a simmer.
Reduce to a low heat and cook for 30-35 minutes, stirring occasionally. Remove from the heat and allow to cool, then puree using a hand blender until you have a smooth sauce.
Cook the Charlotte potatoes in a large saucepan of salted, boiling water for 20 minutes until tender. Drain and run under cold water until they are warm to the touch.
Preheat the oven to 200°C.
Slice the potatoes in half and arrange in a roasting tray. Season with salt and the crushed peppercorns then drizzle with the olive oil. Brush the tuna steaks with the olive oil and season well with salt.
Heat a large frying pan over a high heat until hot and sear the tuna steaks well on both sides for 1 minute. Transfer them carefully so that they are sitting on top of the potatoes in the roasting tray. Roast in the oven for 6-8 minutes until the tuna is cooked and the potatoes are slightly roasted.
Reheat the sauce at this point, stirring in a few strands of lemon zest as you do. Remove the tuna when ready and arrange the potatoes in serving bowls. Spoon the sauce on top of the potatoes, then the tuna on top.
Garnish with the lemon zest, crushed red peppercorns, coriander leaves and a sprig of coriander before serving.
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