Sometimes, when there's leftovers, I don't do more than reheat when I serve them again - if they're stews or soups or a pasta dish, say. Not only will they just taste better because of the extra time, but there's not much else you can do. I do have one soup - turkey/bean/sweet potato - which changes on its own, because the sweet potato cubes fall apart when it's reheated after a day or so, and the nature of the soup changes, becoming thicker, richer, sweeter. And I might add a sprig of cilantro or a squeeze of lime to enhance the change, but that's not really doing anything.
And things like chicken filet or burgers or meatloaf - they turn into lunch, and sandwiches and so on, and that's not really much of a change, to slide the chicken filet between to slices of rye bread and add mayo. It's not really a transformation.
I seem to do most of those with leftover poultry, which is not a surprise. I don't make a lot of beef dishes these days, and when I do, they tend to be either steaks (leftovers, if any, become sandwiches or part of a green salad) or ground beef. Kosher beef is very expensive - I've given up on holiday pot roasts, and don't even think about roast beef. Fish doesn't generate leftovers, and if they do, they're a cold appetizer or something. Dairy and pareve dishes are "made dishes" as they are. I don't change the leftovers - Jonathan takes them for lunch and reheats them.
So it's going to be with leftover chicken or turkey that I will play - leftovers from Sabbath or holiday meals, to be precise, when I might have guests and so make more than people can comfortably eat. There's my "trifecta", for example - Chicken is baked with herbs on Friday. Leftovers are turned into spaghetti sauce on Sunday or Monday, and leftovers of *that* - the sauce and the pasta - are mixed with eggs and baked on Wednesday as a fritatta. There will likely be leftovers of that, but that's lunch on Thursday.
(Chicken spaghetti - make your favorite spaghetti sauce (or, I suppose, heat up your favorite jarred spaghetti sauce) but instead of adding browned ground beef or sausage, put in 1-1.5 cups shredded leftover chicken.)
This Shabbat, we had a guest for lunch, so I made extra chicken - two instead of one, cut into eighths and marinated in lemon peel, lemon juice and chopped parsley, then baked. These produced a huge amount of flavorful juices, so I put them in the freezer, fat and all,in a paper coffee cup. I also had the juices leftover from a turkey breast in there. And I had lots of chicken left over. Lemon chicken does not make great spaghetti sauce, although it could make lovely chicken salad, and I suppose I could have made a non-dairy chicken a la king or something.
Instead, I made hash. Since we'd finished the potatoes I'd cooked for Shabbat, I bought some more and baked them on Sunday night, and let them sit in the fridge until Monday.
Understand - it took me maybe fifteen minutes, if that, to put the marinade together on Friday, and less time than that to prepare the baking pan and arrange the chicken for roasting. The rest took place by itself - the three hours in the fridge, the 2 in the oven. I could go shopping and get lunch and do other things.
Dinner on Monday took me an hour, and that's with nearly everything cooked. I had to debone and shred the chicken - and I put the bones and skin and the leftover juices (except for the fat) in a saucepan with water to cover and let it simmer for a fast chicken stock. I had to cube the potatoes. I had to chop the onion and sauté it in the chicken fat and let it get sweet and slightly brown. I had to combine all these ingredients properly (and I had to wash the pots and utensils and so on as I went, but I used the "pockets of time" when I had no active cooking to do). I enjoyed every minute of it - I like when I have a bunch of things going on at once on the stove - plus I got through two lectures on the English language. Which meant it took about an hour to do, all of it active. I ended by putting the hash in a casserole dish and putting that in the oven.
And I regard it as a transformation, using my leftovers as an ingredient. This is nothing new - there are entire cookbooks dedicated to that, and all purpose ones have all seem to have chapters on that theme, as well. But it is a form of magic anyway.
And tonight? We're having reheated hash. :)