Mushroom Barley Soup

This recipe is adapted from The America's Test Kitchen Family Cookbook
3 tablespoons unsalted butter
1 onion, chopped fine
1 pound cremini mushrooms, stemmed and quartered
1 pound portobello mushrooms, stemmed and cut into 1/4 inch cubes
salt
2 carrots, peeled and chopped
3 garlic cloves, minced
9 cups vegetable broth
1/2 cup pearl barley
1/2 teaspoon dried thyme
pepper to taste

1. Melt the butter in a large stock pot or Dutch oven over medium heat. Add the onion and cook until softened, about 5 minutes. Stir in the mushrooms and 1/4 teaspoon salt and cook until the mushrooms are softened and browned, 10 to 15 minutes
2. Stir in the carrots and garlic and cook for about 1 minute. Stir in the broth, barley, thyme,
   and 1/2 teaspoon salt. Bring to a simmer and cook until the barley is tender, about 50 minutes. Season to taste with salt and pepper before serving.