With their meatlike consistency, portobello mushrooms, served with chard and barley, make a satisfying meal.
Place a medium saucepan over medium-high heat. When hot, spray with cooking spray. Add onions; stir, cover, and cook 1 minute. Add barley; cook, stirring frequently, until barley is lightly toasted, about 8 minutes. Add salt and 1 1/2 cups water. Cover, and bring water to a boil. Simmer until water has been absorbed, about 20 minutes. Remove saucepan from heat, and keep covered.
Heat a large nonstick skillet over medium-high heat. When hot, spray with cooking spray. Add mushrooms, tops up; cover, and cook 8 minutes. Turn mushrooms over; cook 5 minutes more. Transfer to a plate.
Add Marsala to skillet; cook 10 seconds. Increase heat to high; add vegetable stock, rosemary, and pepper. Cook until liquid has reduced by three-fourths and has slightly thickened, about 12 minutes. Add Swiss chard; cook, stirring once or twice, until chard has slightly wilted, 1 to 2 minutes. Transfer chard to a plate. Add butter to pan; swirl until combined. Remove the sauce from the heat.
Cut the mushrooms thinly on bias. Divide barley among 4 plates. Top each with chard, mushrooms, and sauce. Serve.