Mkay, there is only one Way to effectively utilize the energy for induction heating and that is by heating both sides of the electro-magnetic-field, same as pancake-motors really, where you have permanent magnets on both sides of the stator.
So we have to sandwich the coil between two lumps of iron. Solid iron, the ferromagnetic the better. If just heating “one” side we will be wasting half the energy. It really is logical, but sometimes the conventional approach can cloud one’s judgement.
But @robca then it can only be a purely boiling vessel, and we will need to fry separately in a large pan or by some other way.
Of course you don’t need to fry onions for porridge 
The other day I was visiting this street food place we have, since my friend had these coupons. And I was like, why does no one sell porridge. It was all greasy meat and my mouth tasted like raw onions for hours after that 
Something like this. The Induction inserts naturally has the coil in the middle between to iron plates on either side. It could in theory be a PCB coil. To avoid corrosion it will be bonded in there, with just the wires sticking out. Possibly the insert has to be welded to the bottom of the barrel, maybe it can be bonded or brazed, perhaps fastened with a food safe gasket.
Depending on how much surface area, for heating, is really needed, maybe there should be more of them. These things will need to be tried and tested. Of course it is essential that the content is continuously “stirred”, is moving down the center and out across the heat.
