hi
i have done the same thing - the best parting tool i have - its so thin and cuts so easily.
as its hardened steel you will need to use a dremel or similar with a small grinding stone.
just like sharpening a drill bit, the grinder just cuts no problem.
just be careful when you break through , i turned the blade over and then came at it that way rather than straight through from one side.
if you decide to carry on from one side make sure you open the hole up with a fair bit of clearance so the grindstone is unable to jam and possibly shatter.
best of luck
paul