Thanks in advance

I have a blue board wall (Fixed to a stud frame which is then fixed to a brick wall). At the top there is about 150mm of blue board which is not fixed to the frame. Originally i was going to trim the blue board to finish flush with the top of the stud frame, but I am now going to leave the 150mm overhang and fix a piece of timber fascia, butting the timber fascia up under the gutter. (see picture if that made non sense). The blue line in picture shows where the horizontal stud ends, red line is where i will fix the fascia.

My question, since I am not able to fix the timber fascia to a stud, what is my best method to secure the fascia to the blue board. (Fasica will be about 75-100mm) or should i forget this method since i dont have a stud to fix to. If forget it, i am open to other suggestions on how to finish the top???