At present there isn't any simple one-click way of doing this, but it can be done using the advanced search function.
1. Go to the Advanced Search
2. Type the author's name in Search for author
3. Select the forums in which you wish to search.
4. Click the button First post of topics only
5. Click Search
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First hermit: Still there’s one thing about being a hermit, at least you get to meet people.
Second hermit: Oh yes! I wouldn’t go back to public relations.
— Monty Python, The Hermits
(I shall be offline from 27th June until November)