Lay the fish cubes in a large, shallow glass or ceramic dish. In a small bowl, mix together the yogurt, garlic, ginger, chilli powder, ground coriander and cumin. Season the mixture to taste and pour over the fish. Cover and leave to marinate in a cool place while you make the mash.
Cook the potatoes in a large saucepan of salted boiling water for 10 minutes or until tender. Drain in a colander and return to the pan. Mash the potatoes and add the crème fraîche. Continue mashing until smooth, then stir in the chopped coriander, chilli and spring onions. Season to taste with salt and pepper, cover and set aside.
Heat the grill on the hottest setting. Thread the cubes of fish evenly on to 4 metal skewers and cook under the grill for 8–10 minutes, turning once. Serve immediately, with the mash and a green salad.