Start by prepping your green beans. Rinse them good, cut off the stemmy ends, and then cut beans in half to cut down to size. Same thing for washing and prep your mushrooms. Cut into small pieces.
In a skillet add 1 tablespoon of the butter and add your diced onions. You will want to cook them down until translucent. This takes 3 minutes.
Now add in your garlic and mix until fragrant.
Remove your vegetables from the pan and place them into a bowl. Now add your remaining 2 tablespoons of butter into the saucepan.
Use a spatula to scrape down any browned bits in the pan. Once the butter is melted you will add in your flour to create your homemade roux for the green bean casserole.
Whisk in 1 cup of milk into the roux to make a thick paste. Slowly whisk in the broth as well.
Preheat your oven to 350 degrees.
Boil your green beans in a pot of water for 5 minutes. You will then strain and run under cold water to shock the green beans and stop the cooking process.
Pour the beans into the baking dish. Add in your homemade cream of mushroom soup, and then pour in 1/2 cup of milk.
Stir well, and sprinkle with salt and pepper. Cover with aluminum foil.
Cook for 15 minutes until bubbly and heated through.
Remove from the oven, sprinkle the french fried onions on top, and return to the oven uncovered.
Bake another 5 minutes till the fried onions are browned.
Once the green bean casserole is done, remove from the oven, and serve up this homemade side dish!