Melt a big piece of butter in a large pot on medium heat
Add onions, fry for 2 minutes
Add barley, fry for 2-3 minutes, stir almost continuously, coat well with butter
Add 1 cup of white wine, bring to a simmer and reduce heat
Keep adding in ~1/2 cup of liquid every 5-10 minutes (first the rest of the wine, then the broth). When the liquid is absorbed, add more so that there is barely enough simmering liquid around the Barley, stir often.
Separately, lightly fry the mushrooms in butter, when soft, add to the Barley (maybe start the mushrooms at step 4)
It will take a total of approx 40-50 minutes for the Barley to become soft and absorb the liquid. The barley will stay a little crunchy and chewy.
Add some butter and grated parmesan cheese, and serve