Peel and crush (mince) the garlic and peel and finely chop the onion. Melt the butter in a frying pan (skillet), add the garlic and onion and fry very gently for about 10 minutes until the onion has softened but not browned. Season with salt and freshly ground pepper to taste. Pour in the sherry, bring to the boil and continue to boil until most of the liquid has evaporated.
Add the cream and simmer until it has thickened (do not allow to boil). Check the seasoning and then stir in the prawns. Allow them to warm through for a few minutes then pile the mixture into a serving disk and sprinkle with chopped parsley. Serve immediately.