Bright Wizards practically cook themselves. The secret to a tasty Bright Wizard is to make sure he is just a bit overcooked. This means keeping them alive for as long as possible while they burn. You can't just tie them, as they will curl up when their tendons contract, and the insides won't cook thoroughly as a result. You have to make sure to stake them down, limbs outstretched, so that you get an even burn.
(note: found a group of them at the north-east corner of Thorshafn, and killing one gave me the unlock immediately)
Talk to Nathore Gristlechewer at 56700,39400 in Ostland while having a Chunk of Flesh in your inventory. Chunk of Flesh drops from Bright Wizard Acolyte's in Thorshafn in Norsca. The Chunk of Flesh will disappear when you receive the unlock.
This tome unlock contributes to the Ghoulish Gourmet pursuit.
Order: Talk to Bright Wizard Acolyte in Norsca 47.8,12.5 and the unlock.