Melt Unsalted Butter (2 tablespoon) in a 4-to 5-quart pot or dutch oven over medium heat. Add the Onion (2 cup) and sauté for 5 minutes.
Add Button Mushroom (5 cup) and sauté for 5 more minutes. Stir in the Fresh Dill (1 tablespoon), Hungarian Paprika (1 tablespoon), Low-Sodium Soy Sauce (1 tablespoon), and Chicken Broth (2 cup). Reduce heat to low, cover, and simmer for 15 minutes.
Meanwhile, melt Unsalted Butter (3 tablespoon) in a small saucepan over low-medium heat. Add the All-Purpose Flour (3 tablespoon) and whisk continuously until mixture is smooth and golden in color. Add a small amount (about 1 cup) of the simmered broth to the roux, a little bit at a time while continuously whisking.
Pour the Milk (1 cup) into the soup and stir well to blend. Add the thickened roux, stirring continuously until fully incorporated. Cover and simmer for 15 more minutes, stirring occasionally.
Meanwhile, sauté the cup of Button Mushroom (1 cup) in the remaining Unsalted Butter (1 tablespoon) until they are cooked through with crispy edges. If you like, season them with a bit of salt and pepper. Set aside to use as a garnish.
Finally, stir in the Salt (1 teaspoon), Ground Black Pepper (1/4 teaspoon), juice of the Lemon (1/2), Fresh Parsley (1/4 cup), and Sour Cream (1/2 cup). Mix well and allow to heat through over low heat, about 3 to 5 minutes. Do not boil.
Serve immediately topped with Croutons (to taste), sautéed mushrooms, and fresh chopped parsley.