Have the pheasant cleaned and trussed ready for roasting. Heat the butter in a large flameproof casserole or large heavy saucepan. Add the pheasant to the hot butter and brown gently on all sides. Peel and slice the onion and add to the pan. Cover with a lid and cook very gently for 45 minutes.
Pour in the cream, stirring it round the sides of the casserole or saucepan, the strained lemon juice and a seasoning of salt and pepper. Replace the lid and cook for a further 15 minutes. Draw off the heat and lift the pheasant out on to a heated serving plate. Carve the legs from the pheasant first, then carve the breast into thin slices. Strain over the sauce and serve.