Preheat the oven to 200C/400F/Gas 6. Heat half the butter in a large frying pan. As soon as the foam starts to subside, stir in the onion, then cover and cook over a low heat for about 10 minutes or until it is softened but not coloured. Spread the onion over the base of an ovenproof dish large enough to hold the chicken in one layer.
Season the chicken with salt and pepper. Melt the remaining butter in the frying pan and add the chicken, turning once to coat in the hot butter. Brown over a moderate heat for about 5 minutes. Sprinkle with the two tablespoons of paprika.
Remove the chicken from the pan using a slotted spoon and place on top of the onion. Return the frying pan to the heat and add the crème fraîche to deglaze the pan. Heat until just bubbling, stirring and scraping the base and sides of the pan. Taste and season with salt and pepper and add one teaspoon of paprika. Pour the sauce over the chicken and cover the dish with foil. Transfer to the oven to cook for 15-20 minutes. Serve hot.