Hi @greg303,
Considering it is going to be tiled over, the surface doesn't need to be exceptionally smooth, which makes the repair a bit easier.
My suggestion would be to fill the gap with a repair mortar such as this Dingo Fast Set Mortar.
Add clean water slowly and mix until the mortar is stiff enough to stay on a trowel when turned sideways, but still workable. It should be closer to a thick putty than a flowing mix.
You can then apply the mix to the gap, pressing it in so that it sticks firm and then smoothing the surface and shaping the corner.
If there is any inconsistency in the surface, allow it to dry for 10-20 minutes so that it has firmed up a bit, then scrape it back with your trowel.
You should then allow it to dry and cure for at least 48 hours before tiling over the top.
Let me know if you have any further questions.
Jacob
