# What You Need:
→ Vegetables
01 - 1 lb fresh green beans, trimmed and cut into 2-inch pieces (alternatively, 1 lb frozen, thawed)
02 - 1 small yellow onion, finely chopped
03 - 2 cloves garlic, minced
04 - 8 oz cremini or button mushrooms, sliced
→ Sauce
05 - 2 tbsp unsalted butter
06 - 2 tbsp all-purpose flour
07 - 1 cup whole milk
08 - 1/2 cup low-sodium vegetable broth
09 - 1/2 cup sour cream
10 - 1/2 tsp salt
11 - 1/4 tsp black pepper
12 - 1/4 tsp ground nutmeg
→ Topping
13 - 1 1/2 cups crispy fried onions (store-bought or homemade)
# Step-by-step guide:
01 - Preheat oven to 350°F.
02 - Bring a large pot of salted water to a boil. Add green beans and cook for 4 to 5 minutes until bright green and just tender. Drain and immediately transfer to a bowl of ice water to stop cooking. Drain again and set aside.
03 - In a large skillet over medium heat, melt butter. Add chopped onion and cook for 2 to 3 minutes until softened. Add garlic and mushrooms; cook an additional 5 minutes until mushrooms are tender and liquids evaporate.
04 - Stir in flour and cook for 1 minute, stirring constantly. Gradually whisk in milk and vegetable broth. Continue cooking and stirring until sauce thickens, about 3 to 4 minutes.
05 - Remove skillet from heat. Stir in sour cream, salt, pepper, and ground nutmeg until smooth.
06 - Add drained green beans to the sauce and toss to coat evenly.
07 - Transfer mixture to a greased 2-quart baking dish. Evenly sprinkle 1 cup of crispy fried onions over the top.
08 - Bake uncovered for 25 minutes until the casserole is bubbly.
09 - Remove from oven, sprinkle remaining crispy onions evenly, and bake an additional 5 minutes until topping is golden brown.
10 - Allow casserole to rest for 5 minutes before serving.
