This is a particularly tasty recipe I invented when I realised I only had one roasting dish. The vegetables absorb all the lovely (lamb/onion/orange)ness and go nice and crispy.
Put some oil into the roasting tin. Skin the onions, cut them into quarters and put into the tin with the oil. Slice the oranges in half and put juicy side down into the oil. Put the roasting tin into a nice hot oven to warm up. Start some water boiling and peel the potatoes and cut into big chunks (1/2 to 1/4 a potato each). Wash the parsnips, remove the woody core bit and slice lengthwise into quarters.
Put the lamb into the roasting tray and bung in the oven while you start boiling the potatoes and parsnips, give them 20 minutes then drain and add to the roasting dish, ensuring the oranges and onions are on top of some vegetable to give maximum tastiness. After another 40 minutes or so (basting every 10 minutes until everything is cooked, basically), remove from the oven and serve on big plates (don't eat the orange remains...). Take the oily mess left in the dish and use some boiling water and cornflour to make tasty gravy which you should pour liberaly over everything. Serve with a nice red wine and real napkins.