This seems to say that the barley grains were roasted and then ground into flour:
https://www.romanobritain.org/2-arl_food/arl…barley_cake.php
That might work a little better but then it's getting the barley ground into flour which presents its own problems. The Pass the Flamingo says to roast the flour itself:
QuoteMaza begins as álphita: barley flour that has been toasted over a fire,