Try this on scrap:
1. Set the grooving plane for a very light cut.
2. Use a gauge to run lines along the end grain exactly where the grooving plane will go.
3. Very carefully, run the grooving plane for a pass, watching very carefully to make sure you aren’t going outside the gauge lines, etc. You are just trying to get a little bit down into the material.
4. The previous step has left a tiny shoulder defining the groove. Take your tenon saw and run it along that groove, working along the length to get part way down to your depth. Do both sides.
5. Return to the grooving plane to remove the material between the saw kerfs.
6. Light passes with a sharp bench plane when you are all done might tidy things up.

A wide edge like that isn’t going to be easy to run a tenon saw in, so be patient. Make sure your initial groove is deep enough to set the saw. It might take a couple passes with the gauge, plow, back to gauge to feel confident. You may be tempted to use a knife to strengthen that gauge line, but take care. It can work, but it can also slip out and put a cut into the end grain. Also that “clean up with a bench plane” in Step 6 works best when the top line is a perfect as possible, which happens most easily right off the gauge.

Lubricate the plow blade, fence, skate.