Wash and trim 2 lbs. of fresh green beans. Cut the beans into halves or thirds. Bring a pot of water to a boil. Once the water is boiling add the green beans and boil for 3 minutes. After 3 minutes, remove the beans with a slotted spoon or spider strainer and place them into a bowl of ice water.
After the beans are all cooled in the ice water place them in a strainer and drain them. Once drained, place the beans in a 9×13 inch casserole dish.
Cut 1 8 oz. package of button mushrooms into slices.
Heat a skillet to medium and add 1 Tbsp. of butter. Once the butter is melted add the garlic and shallots. Saute for 2-3 minutes, stirring often.
Add the sliced mushrooms along with a generous pinch of salt and pepper. Saute the mushrooms for 8 minutes, stirring often. After the mushrooms are sauteed add them to the green beans in the casserole dish.
Prepare the Cream of Mushroom Soup
To make the cream of mushroom soup, cut another 8 oz. package of button mushrooms into small pieces. You want these mushrooms to be cut smaller than the mushrooms you sliced earlier.
Once the mushrooms are chopped heat another Tbsp. of butter in a skillet. Add the chopped mushrooms along with a pinch of salt and pepper. Saute for 5-6 minutes, stirring often. When the mushrooms have cooked down pour them into a small bowl and set aside.
Reduce heat to medium-low and add 2 more Tbsp. of butter to the skillet. Once the butter is melted add 3 Tbsp. of gluten free flour blend (I like Bob’s Red Mill 1 to 1 flour). Stir the flour into the melted butter, creating a roux.
Slowly add 1 cup of chicken broth to the skillet and stir constantly using a whisk. The mixture will look chunky at this point. Add ½ cup of evaporated milk and continue to stir.
Add ½ tsp. garlic powder, 1 tsp. Worcestershire sauce, and ¼ tsp. salt and pepper to the skillet. Continue to stir until the mixture becomes creamy and thickened.
Assembling and Baking
Once the homemade cream of mushroom soup is ready, pour it into the casserole dish along with the green beans and mushrooms. Stir all of the ingredients together until completely coated.
Place the uncovered baking dish into an oven that has been heated to 375 degrees. Bake for 25 minutes.
After 25 minutes, top the casserole with 1 cup of gluten free French fried onions OR 1 cup of gluten free Panko breadcrumbs that have been mixed with 2 Tbsp. of melted butter. Spread the onions or breadcrumbs over the top and bake for an additional 10 minutes.
Remove the casserole when it is hot and bubbly and allow to cool slightly before serving.
Make it ahead of time: Make this a make ahead green bean casserole by preparing it ahead of time. Simply prepare the casserole as directed and then store to bake when ready to serve.
Enjoy the leftovers: This casserole can be kept in the fridge in an airtight container for 4-5 days after baking.