If you serve a roast (bird or bone-in beef joint) for Sunday dinner, you can cut down on dishes and make your Monday simpler by making pot-au-feu (aka carcass soup) if dinner is prepared in a dish that can be used on the stove and in the oven. Or, if you cooked your roast in the slow cooker, that will work for this purpose too.
Put the (unwashed) roasting pan/slow cooker insert used for cooking the meat in the fridge overnight. You want all those nice juices that congealed on the bottom of the pan; any vegetables or stuffing left behind will add to the flavor of your soup. If your pan/insert is ceramic, bring it to room temperature so it won't explode when exposed to heat. Add enough water to cover the bones, and boil/slow cooker high for 45 minutes to separate the meat from the bones. Strain the soup through a colander, and return the liquid to the cooking pan. Discard any skin, bones, or cartilage; return the meat to the pan.
At this stage, you can add vegetables, like diced onions, carrots, or potatoes to the pot. I usually throw in a bouillon cube and some sage or tarragon, and simmer it for another half-hour, or until the vegetables are tender. The gelatin content of the bones will thicken your soup naturally, but you can also add some cornstarch dissolved in water for a creamier soup. Shortly before serving, you could also stir in some frozen peas or dry noodles. Serve with hearty bread for an easy meal. Plus, your roasting pan will be so much easier to clean!