Trim and blanch green beans. To blanch, boil in salted water for 5-7 minutes until they reach the tenderness you desire. Plunge beans into a bowl filled with ice water. This stops the cooking process and helps preserve the bright green color.If using frozen green beans, skip this step.
1 pound green beans
Sauté onions and mushrooms along with butter in a pan over medium heat.
¼ cup onion , 4 ounces mushrooms, 2 tablespoons butter
Cook until the onions have softened and the mushrooms are golden. Add garlic and cook for 30-60 seconds until fragrant.
2 cloves garlic
Add the flour and stir so everything is well coated. The flour will become absorbed into the butter from the onions and mushrooms. If the flour remains dry, add a tablespoon of butter. Cook over low heat for 2-3 minutes.
2 tablespoons flour
Stir in the stock. Scrape up any browned bits from the bottom of the pan. Bring to a simmer.
¾ cup stock
Add half-and-half and cook over medium-low heat until the sauce begins to simmer and thicken. The sauce is ready when it coats the back of a wooden spoon, and a line drawn with your finger through the sauce holds its shape.Add salt and pepper.
Pour into a lightly greased casserole dish. Bake in a preheated 350°F oven for 25-30 minutes until bubbly.
Top with French-fried onions and return to the oven for 5 minutes.
1 ¼ cups french fried onions
Allow casserole to rest for 5 minutes before serving.
Notes
Cut the green beans into any desired size. I typically leave mine full-size, but you can slice them into smaller pieces.
If using frozen green beans, skip the blanching step. Thaw and drain off any excess liquid, then add to the sauce.
After blanching the green beans, drain them well and then spread them on a clean towel to help remove any excess moisture.
To Make Ahead of Time:Prepare the casserole up to one day ahead of time according to directions, but don't bake. Place everything into your casserole dish. Allow to cool slightly. Cover and refrigerate.Before baking, remove from the refrigerator about 30 minutes prior. Remove cover and bake according to directions. Baking time may increase by 5-10 minutes. Add the fried onions during the final five minutes of baking.