This is a female Slate-colored Junco. Slate-coloured Juncos have variable amounts of brown on their backs and females also have some variable amounts of brown on their head.
Liron had a male Cassiar in his yard. Click the link to see his:
As you can see from his photo it is a male and has a blackish-gray hood that really contrasts with the gray back, wings and flanks which you can see the contrast is noticeably absent on your bird. A female Cassiar would have a gray head that contrasts (to a lesser degree than the male) with the brownish back, flank and wings. Your bird has no brown flanks.