Preheat oven to 350°F. Spray a 2-quart baking dish with cooking spray and set it aside.
Steam your frozen green beans according to your package directions. Drain any excess liquid from the beans and place them into a large bowl. Set cooked green beans aside.
In a medium skillet, over medium-low heat, cook bacon slices until crispy. Remove the bacon from the skillet and let them drain on a paper towel lined plate. Once cooled, chop the bacon into small pieces. Set aside.
Remove all but 1 tablespoon of the bacon grease from the skillet and cook, on medium-high heat, the mushrooms, onions and garlic for 5 minutes or until tender. Remove cooked vegetables from the skillet and set them aside.
Using the same skillet you cooked your bacon and vegetables in, on medium heat, add the butter and allow it to start to melt. Once melted, sprinkle flour in and stir it around in the butter to cook until the flour is a pale brown and no raw flour remains. This will take about 1 and 2 minutes.
Slowly whisk the half & half into the flour and butter. Make sure to whisk continuously to prevent any lumps from forming. Continue to cook until the mixture thickens up. This will take about 3 minutes.
Turn off the heat to your skillet and add the parmesan cheese, Italian seasoning, salt and pepper. Stir to combine.
In the large bowl with your steamed green beans, add the cooked mushroom mixture, chopped bacon and the half & half mixture. Stir to combine all the ingredients then transfer the vegetables to your prepared casserole dish.
Evenly top your green bean mixture with the French fried onions and bake your casserole for 30 minutes or until hot and bubbly.