Paella
Returning to Spain, here's another fantastic "use up whatever's in the fridge" recipe: paella. Paella is more of a technique than a specific set of ingredients, and add-ins can range from rabbit and snails to a variety of seafood. The key to making a good paella is to start with sautéed aromatics and then cook your toasted rice in a flavorful broth.
Add shredded leftover chicken and any other proteins or vegetables you want, keeping in mind the time it takes to cook each addition so that everything is ready when the rice is done. A nice crusty layer of browned rice at the bottom of your pan is the telltale sign of a properly-made paella. When serving, make sure to scrape that crunchy layer off the bottom because it's the best part.