I have a Fissler pressure cooker and I have an electric stove. I turned the heat way down for my dish, but after about 4 minutes, I smelled something burning. I took the pan off the heat, let it come down normally. The onions and mushrooms at the bottom of my pot were burned. I am soaking it overnight, but I'm concerned that's not enough. Should I be doing something else???? It seems like a couple of layers are burned. What's the best cleaning method?