Place the peppers, onion and garlic cloves in a nonstick roasting tin. Roast in a preheated oven, 200°C (400°F), Gas Mark 6, for 40 minutes or until the peppers have blistered and the onion quarters and garlic are soft. If the onion quarters start to brown too much, cover them with the pepper halves and continue cooking.
Meanwhile, heat the oil in a saucepan and fry the ginger, cumin and coriander over a low heat for 5 minutes until softened. Add the potato, stir well and season to taste with salt and pepper. Add the stock, cover the pan and simmer for 30 minutes.
Remove the roasted vegetables from the oven. Place the peppers in a polythene bag, tie the top and leave to cool. Add the onions to the potato mixture and carefully squeeze out the garlic pulp from the skins into the saucepan. Remove the skins from the peppers and add all but one half to the soup. Simmer for 5 minutes.
Purée the soup in a food processor or blender until smooth. Return to the saucepan and thin with a little water, if necessary, to achieve the desired consistency.
Spoon into individual bowls. Slice the remaining piece of pepper and place the strips on top of the soup with a spoonful of fromage frais.