Place barley in a bowl and add 1 cup water; soak for 2 hours or overnight. Drain.
Combine drained barley, apples, 2 cups water, brown sugar, cinnamon, butter, lemon juice, nutmeg, and salt in a saucepan; bring to a boil. Reduce heat to medium-low and simmer until barley and apples are softened and water is absorbed, adding more water if needed, about 1 hour. Remove saucepan from heat and stir walnuts into barley.