Soak the barley in water to cover for at least 2 hours. Drain.

Add the 6 cups water and salt, bring to a boil, cover, reduce heat to low and simmer until tender (about 1 hour). Drain and cool.

Preheat the oven to 350°F (175°C). Grease a 1½-quart baking dish.

Heat the oil in a large skillet over medium heat. Add the onions and sauté until soft and translucent (5 to 10 minutes). Add the mushrooms and, if desired, bell peppers and sauté until liquid evaporates. Remove from the heat and stir in the barley, then the remaining ingredients.

Spoon into the prepared pan. Bake until browned and set (about 40 minutes).