Coat a heavy saucepan with cooking spray and set over medium-high heat; melt butter in pan. Add onions; cook, stirring occasionally, about 5 minutes. Add mushrooms; cook, sitrring a few times, until mushrooms release their moisture, about 5 minutes. Add broth, barley and salt; bring to a boil. Partially cover pan and reduce heat to low; simmer until barley is tender, stirring occasionally, about 60 to 75 minutes.