Shower valve retrofit

[Monica]

We replaced a decrepit 20 yr old shower valve with a sleek Grohe valve and trim. Unfortunately, the new trim is much smaller than the old one, and we now have a 2 inch by 4 inch gap underneath it. The plumber had to leave before he could fill this in, and I think we’d rather do it ourselves anyway. What should I use to fill the hole? The wall under the tile appears to be some sort of very thick cement or mortar

Thanks!