Melt 1 ½ Tablespoons of butter in a saucepan over low heat. Add 3 Tablespoons of all-purpose flour and whisk for 1-2 minutes, stirring constantly.
Whisk in 1 ½ cups of milk and bring to a boil. Continue to cook, whisking constantly for 1-2 minutes or until the sauce starts to thicken.
Whisk in the 4 teaspoons of Hidden Valley Original Ranch Salad Dressing and Seasoning Mix Packet, 1 teaspoon of salt, and ¼ teaspoon of black pepper. Remove the sauce from the heat and set aside.
In a large skillet, melt the remaining ½ Tablespoon of butter. Add in the 1 cup of diced onions and 2 cloves of minced garlic. Sauté for 2-3 minutes until tender and then remove half of the mixture into a bowl and set aside.
Add 1 ½ cups of sliced mushrooms to the skillet along with the remaining sautéed onion and garlic mixture. Mix well and cook on medium heat for 5 minutes or until the mushrooms are tender. Once done, remove from heat.
Add the cooked green beans and mushroom mixture to the large saucepan containing the white sauce. Using a large spoon, mix well to combine.
Mix the bite-sized cut bread to the remaining onion and garlic mixture. Mix well to combine. Set aside.
Pour the green bean casserole mixture into the 9x13 baking dish.
Sprinkle the breadcrumbs over the casserole mixture and spread evenly.
Bake uncovered until done, about 30 minutes.
When done, let the casserole rest for 5 minutes before serving.