Melt 4 Tablespoons butter in a large pot over medium heat. Sauté 2 cups onions in the butter until they just start to brown; about 5 minutes. Add 8 ounces sliced mushrooms and sauté for 5 more minutes. Stir in 2 teaspoons oregano, 1 Tablespoon sweet paprika, good dash or smoked paprika, 1 Tablespoon soy sauce and 2 cups broth. Reduce heat to low, cover, and simmer for 10 minutes.
In a separate small bowl, whisk 1 cup milk and 3 Tablespoon flour together. Pour this into the soup and stir well to blend. Cover and simmer for 10 more minutes, stirring occasionally.
Finally, stir in 1 teaspoon salt, ground black pepper, 2 teaspoons lemon juice, ¼ cup parsley and ½ cup sour cream. Mix together and allow to heat through over low heat, about 3 to 5 minutes. Do not boil.
(If the soup seems more thick than you want, add another ½ cup vegetable broth.
Serve immediately with 1 ½ cups cooked rice and extra sour cream to stir in if desired.