Remove and discard tough outer leaves of lemon grass, saving about a 3-inch section of tender white root. Slice root in half lengthwise. Lightly crush lemon grass, using the side of a knife.
Heat oil in a Dutch oven over mediumhigh heat. Add lemon grass and onion; sauté 3 minutes. Stir in 1/4 teaspoon salt, turmeric, ginger, and red pepper. Sauté 2 minutes or until vegetables are tender. Add sweet potato, baking potato, carrot, and mushrooms. Sauté 2 to 3 minutes or until vegetables are lightly browned. Stir in broth and water; bring to a boil. Cover, reduce heat, and simmer 23 to 25 minutes or until potato and carrot are tender.
Remove lemon grass. Stir in peas, coconut milk, and 1 teaspoon salt. Cook, uncovered, 2 to 3 minutes or until peas are thoroughly heated. Stir in cilantro.