One of the most important maintenance items around the house is to keep the cooking pans in perfectly clean condition. I consider this to be by far the most important DIY item to stay healthy at home.
On occasion I find there is some particularly resilient residue.
The white layer you see in the picture has the habit of showing up after cooking beans or lentils. While boiling anything else, it's enough to make sure there is a reasonable amount of water left in the pot for it to remain easily cleanable, but beans and lentils leave this particular residue. This suggests that it is not caused by water hardness.
What is this residue and what is an effective way to get rid of it?