Heat butter in a large pot and add the onions. Cook over medium heat, stirring often, until the onions are soft and just beginning to brown; about 7-8 minutes.
Add whit wine to pot. Add the mushrooms and spices. Slightly lower the heat, cover, and cook 5 minutes, stirring frequently.
Add the soy sauce and stock. Cover and simmer about 10 minutes.
In a separate small pan, mix the 1 tbsp butte and flour to form a thick paste. Cook, stirring constantly, for 1 minute, then whisk in the milk and cook over medium heat, stirring frequently, until steamy and slightly thickened.
Add the milk mixture to the soup; add the lemon juice to the soup. Stir well; serve.