Use the boiled water but add liquid detergent to it. If that leaves some, then get some (sorry Aunty) Gumption, the paste cleaner for such applications. You may need to go to a Hardware store for it. I usually put about 2cm of water in my pans, get it boiling, turn the heat off and then some dishwashing detergent and leave overnight to help soften and soak off burnt on bits.