How do I avoid a burnt underlayer with pressure cookers?
When we use a pressure cooker, we do so with separate pans inside the cooker - flat round tins, in our case - that can be kept off the bottom by adding something underneath.  Or stack up cook several things at the same time, if the size allows.

I recall seeing these largish round metal rings like an inch high that I think were for that, or (inverted) small steel plate or bowl.  Since there's no (or little) direct contact the bottom doesn't overcook. I imagine some kind of rack or other insert would also work.