Well, if no-one else is going to suggest anything, it depends on whether you
want to hide it as much as possible. I guess you mean 3.5 mm deep and around
1 cm wide? Is it on a horizontal surface? Araldite or similar should stick
provided the surface is reasonably clean. If it's a horizontal surface you
can warm with a hot air gun or even a hair dryer after applying, this will
make it flow and self-level just before setting (have a practice first).
Plain epoxy will set brown and translucent, but you can often fake stone /
ceramic by filling with a suitable white or coloured powder, polyfilla or
similar will do. The more filler you use, the better it will look, but the
weaker it will be, also it won't self-level but it will be easier to sand.
If it's a potential leak path you need to seal it well; if not, then
strength doesn't matter so much.
You could try the Colourfill resins used for joining and repairing kitchen
Car body filler will probably also stick OK and can be easily sanded after
it sets, but it's usually grey.