Heat 2 tbsp butter in a medium pot. Add onion and sauté for 3-4 minutes until softened. Add 1/3 of the sliced mushrooms, 1 cup of broth, soy sauce, paprika and dried dill. Cover and simmer on low for 15-20 until everything is soft.
Meanwhile, melt the remaining butter in a large skillet until foamy. Add remaining sliced mushrooms and sauté over medium-high heat to brown the mushroom. Reserve about a 1/2 cup for garnish if desired.
Using a blender or handheld emersion blender puree the cooked onion and mushrooms until smooth. Return mixture to the pot and stir in remaining broth. Whisk together milk and flour and stir into the blended mixture. Bring to a simmer and add sautéed mushroom, 1/2 tsp salt and pepper. Cook covered for 15 minutes, stirring occasionally.
Stir in sour cream, lemon juice and additional salt to taste. Serve garnished with reserved mushrooms, fresh dill or parsley if desired.