Melt butter in a medium saucepan over medium-high heat. Add shallots; cook 1 minute, stirring occasionally. Stir in mushrooms and garlic; cook 2 minutes, stirring occasionally. Stir in barley; cook 1 minute, stirring constantly. Stir in broth, pepper, and salt. Bring to a boil; cover, reduce heat, and simmer 15 minutes or until liquid is absorbed. Stir in the spinach; cook 1 minute or until the spinach wilts.