Boil the aubergines in water with 1 teaspoon salt for 5 minutes. This will prevent them from absorbing too much oil. Drain them completely.
Heat the olive oil in a medium pan. Add the spices and cook for about 10-20 seconds, being careful not to let them burn.
Quickly add the garlic and aubergines and stir fry for 1 minute.
Add the tomatoes, and sultanas and season with salt and pepper. Cook uncovered until the tomatoes have softened and the mixture has thickened. Add the herbs. The mixture is best if it can sit around for an hour or more to let the flavours develop and mingle.
To serve; spoon a little of the mixture onto each crostini and top with 1/2 teaspoon crème fraîche and a mint or coriander leaf.
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