What kind of onion are you shaving with? Never heard of such.
Sharpening with whetstones involves a little bit of a learning curve, most start with less than stable hands but getting to a proficient edge is quickly learned. It would not take long before you can sharpen as good as if not better than the gizmo edge.
Gizmo sellers will not tell you about the importance of thinning your blade to maintain good cutting properties in food. If a knife only sees gizmos it will quickly develop thick shoulders that may shave hair just fine but won't cut a carrot worth a damn. Other things gizmos won't do is properly sharpen an asynchronous edge and of course single bevel edges.
To test the cutting properties of an edge I like to cut scallions for a clean cut without accordians, dice onions to ensure tip is sharp and thin, cut carrots and observe any splitting noise to check thinness. In other words to test "sharpness", I cut food. If I have a reason to think I may have a ding in the edge, I may use paper to identify if and where.