To make green bean casserole with sour cream, combine parboiled green beans, sautéed mushrooms, flour, chicken stock and milk with the sour cream in a casserole dish. Top with Parmesan cheese, paprika, soy sauce, pepper and caramelized onions and bake for 15 minutes until bubbly.
To make green bean casserole using sour cream, first butter a casserole dish, then dust it with grated Parmesan cheese. Caramelize 2 medium onions for 30 minutes over medium-low heat in 2 tablespoons of vegetable oil, then set aside. Boil 1 1/2 pounds of green beans in salted water for two minutes, then drain and plunge into a bowl of ice water, drain, and set aside. Preheat your oven to 450 degrees Fahrenheit.
Sauté 1 pound of mushrooms in 2 tablespoons of vegetable oil over medium-high heat for five minutes, then sprinkle with 2 tablespoons of flour, and stir with a wooden spoon to form a paste. Pour in 1 cup of chicken stock, lower the heat and simmer for two minutes before stirring in 1 cup of sour cream and 1/2 cup of milk. Simmer and stir occasionally for five minutes, then pour the sauce into the bowl of green beans. Top the mixture with 1/4 cup of grated Parmesan, 1/2 teaspoon of paprika, 1 teaspoon of soy sauce and 1/4 teaspoon of black pepper. Place the mixture in the casserole dish, top with the onions and 2 tablespoons of butter, bake for 15 minutes, then serve.