Cut squash in half lengthwise and place cut side down on a half sheet pan. Bake until soft, 30 to 35 minutes. When cool, scoop out flesh and purée in a food processor.
Combine the squash purée, water, barley, butter and salt in 2-inch deep full hotel pan. Mix until well combined. Cover tightly with plastic wrap then foil. (You want the pan to be airtight to keep in the moisture.)
Bake for 1 hour. Uncover and stir. It should be creamy and barley should be soft but not mushy. If it seems too watery, recover and bake for 20 minutes more.
