Preheat oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3). Line a baking tray with baking paper. Wash sweet potatoes thoroughly and cut them in half lengthwise. Brush the cut surfaces with 1-2 tbsp. oil and season with salt and pepper.
Place on the sheet with the cut surface facing upwards. Bake in a hot oven for 30-40 minutes until the potatoes are soft.
For the salad, drain the tuna and pluck it finely. Peel onions and cut or slice them into thin rings. Wash the coriander and dab dry. Coarsely chop the leaves. Cut the chilli lengthwise, remove seeds, wash and chop finely.
Mix with onions, coriander, tuna, lime juice and 2 tablespoons of oil. Season with salt and pepper.
Spread the tuna salad over the potatoes. Stir the crème fraîche until smooth and put 1 blob on each potato. Sprinkle with pepper.