Bring a large pot of lightly salted water to a boil. Add rice noodles and cook until tender yet firm to the bite, about 3 minutes. Drain and rinse well with cold water to stop the cooking; set aside.

Heat oil in a large saucepan over medium heat. Stir in garlic, lemongrass, and ginger; cook and stir until aromatic, 30 to 60 seconds. Add curry paste and cook for 30 seconds. Pour in about 1/2 cup chicken broth and stir until curry paste has dissolved, then add soy sauce, sugar, and remaining chicken broth. Bring to a boil, then reduce the heat to medium-low. Cover partially and simmer for 20 minutes.

Stir in coconut milk, spinach, shrimp, mushrooms, cilantro, and lime juice. Increase the heat to medium-high and simmer until shrimp are bright pink on the outside and the meat is opaque, about 5 minutes.

To serve, place some rice noodles into each serving bowl and ladle soup over the top. Garnish each bowl with a sprinkling of sliced green onion.