Warm the oil in a large stockpot over medium heat. Add the onions, garlic, and gingerroot, and saute, stirring often, until the onions are soft and golden, about 10 minutes. Add the mushrooms, then saute for 5 minutes. Sprinkle on the coriander, cumin, turmeric, and cayenne, and cook for 2 minutes, stirring often. Pour in the vegetable stock, coconut milk, and salt, then bring to a boil. Add the potatoes and cook for 10 minutes, stirring occasionally. Mix in the green beans, red bell pepper, and noodles, and cook at a lively simmer until the vegetables and noodles are tender, about 10 minutes. Stir in the lemon juice just before serving.