Make a delicious green bean casserole from scratch! This comforting Southern side dish is made with a creamy homemade mushroom sauce and canned green beans, then topped with crispy fried onions. It’s the perfect addition to all your Thanksgiving and Christmas gatherings.
Preheat oven to 350°F degrees and grease a 9x13 casserole dish.
To a large dutch oven, melt butter over medium-high heat.
Add mushrooms and shallot, and cook until mixture is brown and caramelized - about 7-10 minutes. Add garlic and stir quickly to avoid burning.
Transfer the mixture to a plate, and do not wipe or rinse your dutch oven.
Add butter to the same dutch oven over medium-high heat.
Then, add flour, whisking constantly. Then, gradually add in milk and whisk until smooth. Continue cooking until the flour, milk, and butter thicken and begin to bubble - about 5 minutes.
Remove from heat and add the Parmesan cheese, kosher salt, and pepper. Stir until melted and smooth.
Stir in green beans, mushroom mixture, and water chestnuts.
Transfer the mixture to the 9x13 casserole dish.
Bake, uncovered, until bubbly - about 15 minutes.
Remove from the oven and top with the fried onions and continue baking until the topping is golden brown - about 10-15 minutes.