No need for a 1/2 inch notch, go down to 1/4x1/4 inch. Small finish nails can be drove into some of the joints to help keep the sheets from sliding. If this is a wet area, use thinset. If this is a backsplash or wet area, use mastic instead. It tends to slide less. There are some non sag thinsets on the market also that would help.
