SPOILERS
You get 11,500 experience for creating the stew (at the stove on level 1).
You get an additional 18,750 experience for placing the stew in the tunnel (which lures the Umber Hulks to that area).
You get additional experience for killing the hulks, if you choose to do so.
You can kill the hulks first, but you still get the experience for "luring" them down there, which seems to be an oversight on the part of the programmers. I don't see why there should be a particular experience point gain just for putting the stew in the tunnel otherwise. It doesn't seem that Kevka was wrong, since what he said was
I made the stew and lured the Umber hulks into the prisons
I got EXP
Then I opened the prison, attacked and killed them
and I got yet more EXP
Which sounds pretty accurate to me.