I will have egress windows cut into my cement basement walls for a
major remodel, will use a standard 48" x 48" slider or casement.

Two contractors told me they simply frame the new concrete opening
with 2 inch "green pressure treated lumber" and then set the window.
There may be a bit more lumber for the header "to align with basement
ceiling" or structural needs, but other than that all there will be is
a single board between the cement and window frame.

(FYI... my city does require an engineers document specifying any
structural header needs in order for the city to give me a permit to
cut two big holds in my basement wall. So that bit will be taken care
of.)

But just a single 2" board sounds too fast and easy. Is there a
"proper" way or am I just use to doing things the hard way?

Thx
Dave-in-Denver