...forthcoming from Naxos:
https://www.mdt.co.uk/onslow-mendelssohn-kalliwoda-romantic-viola-hiyoli-togawa-lilit-grigoryan-naxos.html (https://www.mdt.co.uk/onslow-mendelssohn-kalliwoda-romantic-viola-hiyoli-togawa-lilit-grigoryan-naxos.html)
Something to look forward to.
BTW: In the "more info" section they speak somewhat misleadingly of an "arrangement" for viola of Onlsow's cello sonata. This "arrangement" is by Onslow himself and appears to be part of the earliest editions (see IMSLP). It is a case rather like Brahms' clarinet / viola sonatas.
As far as I know there are recordings of the entire op. 16 played on the cello--and also on the violin (jittery and unattractive on period instruments)--long before any of Onslow's "true" violin sonatas were recorded.