Mix sweet potato, oil and salt and pepper. Toss and place on baking sheet.
Bake at 400 deg for 30 minutes.
Melt butter in large dutch oven over med-high heat. Add the onions. Cook until softened, about 5 minutes. Stir in mushrooms and 1/4 t. salt. Cook until they have softened and browned, 10 - 15 minutes. (This salt is necessary to get the liquid out of the mushrooms.).
Stir in the carrots and garlic. Cook for 1 minute. Stir in the broth, barley, lentils, thyme and 1/2 teaspoons salt (if desired). Bring to simmer. Cook until barley is tender, about 50 minutes. Season with salt and pepper before serving.
Divide into 2 oven safe bowls and top with the sweet potatoes.