Try boiling a kettle, filling a bowl of water, pop it in the oven on 100 for 30 minutes. That will soften it up. Then you could also try a paste of bicarbonate and water painted on and leave for a while. It might take several goes.
When it’s done, buy a silicone liner for the bottom and wipe out the sides and roof with soapy water every time you use it. It saves sooo much time in the long run.