Sounds like you are onto a very good mulch with the leaves and stalks!

Yeah the grass sod should be very good! Alan Chadwick calls this turf loam. His method is alternating sod soil touching soil, grass touching grass. (I wasn't as precise when I made mine) With our heavy clay soil I can hardly call this result loam. But it is very enriched soil from all of the composted organic matter of grass, roots and weeds.

I am very confident I can plant directly into this bed any type of vegetable. Disclaimer--- the soil under the grass was healthy to begin with, but now its even better.