Transfer the food. The metal is not worth it.

Transfer the food. Stainless steel has no business in a microwave, and the lunch box is not the exception.

Here is the short version: metal reflects microwaves instead of letting them pass through. That can cause sparks (arcing) inside the oven, leave scorch marks on the steel, and in bad cases damage the magnetron — the part that actually makes the microwave work. Even if it doesn’t visibly spark, the food inside won’t heat evenly anyway because the microwaves aren’t reaching it. So you get the downside of a potential fire hazard and the downside of a cold middle. Great combo.

There are some microwaves with metal racks and some special “microwave-safe metal” containers, but a typical stainless steel lunch box is not engineered for that. If the manufacturer hasn’t explicitly printed “microwave safe” on the bottom, assume it isn’t.

Reheating in a glass or ceramic dish takes thirty seconds and costs you nothing. That is an easy trade.

Your microwave is more expensive than the time it saves you.