The best leftover strategy begins before the burger goes into the refrigerator: separate wet, cold and crisp components from the patty and bun.
Disassemble when practical
Remove lettuce, tomato and pickles. Store the cooked patty in a shallow covered container and the bun separately. Refrigerate promptly within the applicable food-safety time limits.
Reheat the patty gently
A covered skillet over medium-low heat preserves texture better than blasting the patty. Add a teaspoon of water beside the patty and cover briefly to warm the center and remelt cheese. Reheat leftovers to 165°F (74°C).
Refresh the bun
Toast the cut sides in a dry pan or warm the bun briefly in an oven. A microwave can make bread temporarily soft and then tough, so use short intervals if it is the only option.
Rebuild with fresh contrast
Use fresh lettuce, tomato or pickles rather than reheating them. Add sauce after the hot components are ready. The leftover burger will not reproduce the original crust perfectly, but it can still be balanced and enjoyable.