Basically a fritter made entirely from leftovers, bubble and squeak epitomizes resourcefulness. The dish repurposes leftover mashed potatoes and leftover vegetables. English versions usually contain cooked cabbage, but not always — and therein lies the dish’s defining characteristic: versatility. You can add just about anything you have in the refrigerator to bubble and squeak and make it work if you have enough mashed potatoes to bind it. Cook it to 165 degrees Fahrenheit, the minimum temperature for reheating leftovers safely.