I do what Dave C recommended in one of his many videos.
Grab a container, mines a chinese takeaway tub, pull some water into it from the hot water tap, get a group head brush, wet it and brush the gasket all around. Clean the brush and repeat. Then pull some water through the group as you have done. You could go one step further and put your spare portafilter with blank basket and do a water only backflush., which I do after the last coffee of the day.