A marshmallow that has been roasted over an open flame.
A popular camping or backyard tradition in North America is the roasting or toasting of marshmallows over acampfire or other open flame.[SUP][5][/SUP] A marshmallow is placed on the end of a stick or skewer and held carefully over the fire. This creates a caramelized outer skin with a liquid, molten layer underneath. According to individual preference, the marshmallows are heated to various degrees—from gently toasted to a charredouter layer. The roasted marshmallow can either be eaten whole or the outer layer can be removed andconsumed separately and the rest of the marshmallow toasted again.[SUP][citation needed][/SUP]
Please remember to capitalise the name of TV shows etc. It's called The Mentalist.
Also try not to post potentially offensive words in full. Use asterisks as I have shown above in red. We will know what the word is meant to be.
Of course, the speaker did not actually want the "b*tch" to be stuck on a fork and held over an open flame. The context would help but I imagine that the speaker wanted the "b*tch" to be interrogated very strongly, or to be charged with a crime and then hopefully sentenced to a long period in prison.