Begin by placing the salt in a mortar and crush it quite coarsely, then add the garlic and, as you begin to crush it and it comes into contact with the salt, it will break down into a purée.
Next, add the ground ginger, turmeric and the oil, and using circular movements, really work all the ingredients together until you have a thick brown paste.
Next, scatter the chopped onion in the middle of a small roasting tin and then, holding one of the chicken breasts in your hand and using a spatula, smear half of the paste over both sides of the chicken breast and then place it on top of the onion before doing exactly the same with the other.
Now transfer the tin to the oven and cook the chicken on the middle shelf for 30 minutes.
Meanwhile, in a jug, whisk together the crème fraîche and the curry powder.
Then, when the chicken has had its cooking time, spoon the crème fraîche over the chicken breasts and cover the tin with foil before returning it to the oven for a further 30 minutes.
I like to serve this with spiced basmati rice and add a few coriander sprigs for a garnish.