Green Bean Casserole with Mushrooms

Ingredients:

Casserole

• 2 tablespoons unsalted butter
• 1 white onion, diced
• 110 g cremini mushrooms, thinly sliced
• 110 g shiitake mushrooms, thinly sliced ​​(remove stems)
• 2 tbsp. flour
• 2 cups lightly salted chicken or vegetable broth
• 0.5 cups cream with 10% fat content
• 700 g green beans, trimmed and halved

Topping

• 1 head of white onion, sliced ​​into thin half rings
• 1 cup of sour milk or kefir
• 2 cups of flour
• Vegetable oil, for frying

Preparation:

1. Casserole:
   
   In a large Dutch oven, melt the butter over medium heat. Add the diced onion and cook, stirring, until softened, 7 to 10 minutes. Add the mushrooms and cook until softened and most of the liquid has evaporated, 10 minutes. Sprinkle with flour and cook, stirring, for 1 minute. Add the broth and cream. Bring to a boil, then reduce the heat to medium and simmer the sauce until thickened, about 5 minutes.
2. Bring a saucepan of water to a boil and add salt. Add the green beans and cook until crisp-tender, 7-10 minutes. Drain, shake off excess water, and add them to the mushroom sauce. Cook, stirring occasionally, until the beans are very tender, 10-12 minutes. Season with salt and pepper to taste.
3. Topping:
   
   Soak the chopped onion in the sour milk for 5 minutes. In a medium bowl, combine the flour with 1 teaspoon of salt. In a large saucepan, heat 2–3 inches of vegetable oil to 350°F (175°C) on a deep-fry thermometer. Remove the onion in small portions from the sour milk, let the excess drain, then dredge in flour and shake off any excess. Fry until golden brown, 4–6 minutes. Transfer the fried onions to paper towels to drain off excess oil.
4. Transfer the green bean mixture to a 4-quart baking dish and top with the fried onions. The casserole and topping can be made up to 2 hours ahead; refrigerate the casserole. Bring to room temperature; bake in the oven at 350°F (175°C) for 30 minutes. Top with the fried onions; bake for an additional 5 minutes.