Preheat the oven to 350°F.
In a large pot of boiling water, cook the green beans for 5 to 6 minutes. Immediately transfer them to a large bowl of ice and cold water to cool. Set aside.
In a large skillet, sauté the chopped bacon until fully cooked. Remove from the pan, keeping the remaining bacon fat in the pan. (If there isn't much fat in the pan, you can add a few teaspoons of olive oil.)
Add the onions and garlic. Stir until soft and fragrant. Then, whisk in the flour to made a creamy roux.
Pour in the milk and broth, and simmer until you have a thick texture. Toss in the mushrooms and half of your cooked bacon and cook on low heat until the mushrooms are soft.
Stir in the rosemary, parsley, thyme, granulated onion, granulated garlic, salt and pepper until combined. Remove from heat.
Stir the blanched green beans into the cream sauce until well coated all over. Pour into a deep rectangular baking dish, and cook for 25 minutes.
In a large skillet over medium heat, add the breadcrumbs and butter. Stir until the breadcrumbs are toasted and golden.
Remove the casserole from the oven and top with a layer of cheese, remaining bacon, breadcrumbs, then crispy onions! Bake for another 10 minutes! Serve hot