Melt butter in large pot over medium heat. Add onion and cook for 5 minutes. Add mushrooms, chopped garlic and cook 5 minutes more.
Stir in broth, paprika, soy sauce and dill. Reduce heat to low and simmer, covered, for 15 minutes.
Whisk together milk and flour in a small bowl. Stir into soup. Simmer, covered, stirring occasionally, 15 minutes more.
Stir in sour cream, parsley, lemon juice, salt and pepper. Simmer, covered, until heated through, 3 to 5 minutes.
Garnish with parsley, sauteed mushrooms (if using) and croutons.