Ah, I see.
So you could mill it horizontally with a large diameter cutter / fly cutter . . . . or in its position shown, vertically, using a long cutter (but you'd probably get chatter).
If I still had access to a surface grinder you could skim that off in a matter of minutes using a mag-base.
I don't know anyone with a surface grinder, now, though.
Will make some calls about the plate, later.