Not leftover pasta sauce, but pasta sauce made with leftovers and uncooked items of your choice. The tidy way is to make a white sauce, add the leftovers and spices, and let the mixture simmer for 10-15 minutes, checking every now and then to see if it needs water. You can melt cheese in it towards the end, or you can sprinkle grated cheese on the cooked pasta before pouring the sauce. You can, if so desired, make the white sauce while you have the ingredients already in the pot.