Drizzle a little of the oil over each sweet potato, then rub it all over the skin. Put the potatoes on a baking sheet. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 25–30 minutes until tender and cooked through.
Meanwhile, place the prawns and avocado in a bowl and toss together. Mix the mayonnaise with the milk or water until smooth, then add the crème fraîche and tomato purée and mix until well blended. Add the prawns and avocado and toss well to coat lightly.
Remove the sweet potatoes from the oven. Split the hot potatoes and fill them with the prawn mixture. Serve garnished with a pinch of paprika and some freshly cut sprouting seeds.