Preheat the oven to 375°F. Spray an 8 x 12 inch baking dish with cooking spray and set aside.
Place the frozen green beans into a microwave safe bowl and microwave for 8 minutes. Drain well then set aside to cool slightly.
In a large skillet, cook the diced bacon and onion together. Season with salt and black pepper to your taste.
Cook over medium-high heat until browned and fully cooked. Add 1/2 cup all purpose flour to the pan. Stir until it resembles a paste. If needed, add chicken broth, additional butter or bacon fat to thin. Add salt, lemon pepper, granulated garlic, onion powder and herbes de provence.
Slowly pour in the chicken stock and milk, stirring constantly until the flour is dissolved and no lumps remain. The sauce will immediately begin to thicken. Taste and adjust the salt and black pepper to your taste.
Add the green beans, mushrooms and 1 cup Parmesan cheese. Stir until evenly coated.
Pour into the prepared baking dish.
To make the topping: Finely dice the bacon by hand or in a food processor. [Partially frozen bacon makes this task much easier.]
Toss with the panko breadcrumbs and Parmesan cheese.
Sprinkle evenly over the top of the casserole. Sprinkle with sliced almonds.
Spritz the top lightly with cooking spray or spray oil.
Bake for 35 minutes. Place aluminum foil over the top to prevent over browning if needed. Allow to rest on the counter for 5-10 minutes prior to serving.