Thinly pare the rind from 1 orange, and cut the rind into thin strips. Grate the rind from the other orange, and squeeze the juice from both.
Heat the butter and oil in a frying pan, add the chicken pieces and brown on both sides. Remove from the pan and keep warm. Add the onion to the fat remaining in the pan and gently fry for 5 minutes.
Drain off any fat, then add the Worcestershire sauce, water, tomato paste, orange rind (strips and grated) and juice. Bring to the boil and season lightly with salt and pepper. Line a shallow ovenproof dish or roasting pan with foil, leaving enough on either side to over lap in the middle.
Place the chicken on the foil, spoon over the orange sauce, then cover with the foil, securing the edges by folding together. Bake at 180 C for 1 hour. To serve, spoon the juices over the chicken.