Add a little oil, and a few minutes in a frying pan or microwave will do it. Maillard reaction (browning) starts at around 140°C and shiitake aren't exactly thick, so they won't take much longer than it takes to get some extra colour on them. Average frypan and oven temp is usually around 180°C, so it's not something you really need to think or worry about.

They also think you need a certain hypersensitivity for this to happen. If this were a significant risk, there would be huge amounts of cases in East Asia. This case became a science tabloid spam piece because it's so unusual.