Hello Seegull, its not so much about “building soil” as growing whatever is happy in the surface-mulch conditions.
So it depends on what compost you have used to mulch. If the top 5cm (@in) or so is fine, you can sow or plant any veg in the first year, or at least I do, with good results. I recommend squash for where the mulch is more lumpy, even for planting through holes in polythene.
