Search for all posts started in a category by user

i'd like to search all the progress diary threads started by particular users, but can't seem to find a way to do it. i've tried by clicking on their username (no option) as well as through the advanced search feature (again, no option for threads started by that user in a particular category).

i was thinking about doing individual threads for our progress diaries to make it easier to read through comments for that particular post (it's a 'pain' to jump pages and feel you're missing some comments). now, i'm thinking we'll just have to do one uber long thread. maybe not, though. maybe by the time i go to post an update, i'll have realized that there is actually a way to do it. we all know by now how 'gifted' i am when it comes to the computer and figuring out soft mozart lol