it`s probably oxidised Fats that are causing it, they`ll trap food particles and smell as they decay.
I`de boil water in my largest SS pot with a little caustic soda in there and pour that down it to fill it up with it switched on.
you`ll probably need to do this a couple of times, being carfull not get it in your eyes!
you don`t want a Strong soln of it, dillute and plenty HOT should be fine.
failing that, spray Oven cleaner down it and leave it a while, then do the hot water thing (plain water just Hot).
So long and Thanks for all the Fish ;)