Bring a large pot of water to a boil; add noodles and cook about 3 minutes or until tender. Drain; hold in cold water. In large pot, bring stock and curry sauce to a simmer, whisking to blend. Remove from heat; add fish sauce and lime juice.
For each serving, to order, heat 1 cup soup base with 1/4 cup noodles, 1/4 cup mushrooms, 1/4 cup chicken, 2 tablespoons bean sprouts and 1 tablespoon green onions. Garnish with cilantro leaves.