"Leave out the tomato puree' eintirely, if you like. The peppers need liquid to continue cooking in the pressure cooker, so when you are softening them see how much liquid they release. If the liquid fills the bottom of the pan and comes up about ¼" or 1 cm that is plenty for the cooker to reach pressure. Otherwise, make up the rest with the tomato puree' right before you close the lid...."