I save the left over veges in a large freezer bag or container in freezer. Great way to make vegetable soup! Your veges are already seasoned with butter or we use bacon drippings for green beans. Our vegetable soup is just that. No meat added - came from a large Catholic family and this was our Lenten meatless Friday dinners.
Just add potatoes and tomato sauce and cook til taters are done. If I know I have enough veges in freezer for soup I add a few extra potatoes to my roast. This way your potatoes are already done and the soup is just a matter of heating it all through. you could always use leftover roast but we usually make sanwhiches, meat gravy and noodles, or barbq beef sandwhiches with the leftover meat - you can also use the beef to make philly steak sandwhiches just add green peppers and onions and chesse - great on garlic texas toast. ( I have used leftover hotdog buns for garlic bread or buns) Gee my list goes on lol