Preheat the oven to 400°F.

Spray an 8- or 9-inch square baking dish with nonstick cooking spray. Set aside.

Bring a large pot of salted water to a boil.

Cut the green beans into 1 1/2-inch pieces. Slice the mushrooms, finely chop the onion, and mince the garlic.

Once the water is boiling, add the green beans. Blanch until they are bright green, about 3 minutes. Drain in a colander and rinse with cold water.

Set a large pot over medium-high heat and add the bacon. Cook, flipping once, until browned and crispy. Transfer bacon to a paper towel-lined plate to drain. Do not discard the fat from the pot.

Add the mushrooms and chopped onion to the bacon drippings. Saute until mushrooms are browned and shrunken and onion is soft, 6-7 minutes. Add the garlic and thyme and cook until fragrant, about 30 seconds.

Sprinkle the flour over mushroom mix and stir with a spatula to coat. Slowly pour the chicken broth into the pot, whisking to dissolve flour. Pour in the half-and-half, and season with the salt and black pepper.

Cook the sauce, whisking constantly, until it begins to bubble and thicken. Reduce heat to medium-low and allow sauce to simmer, whisking occasionally, until it is reduced by about a third, 5-7 minutes.

Crumble or chop the bacon into 1/4-inch pieces. Add the green beans and half the bacon to the pot, and stir to coat with the sauce. Pour the green bean mixture into the prepared baking dish.

Bake the casserole on middle rack of oven for 15 minutes.

Open the oven and sprinkle the french-fried onions and reserved bacon in an even layer over the top. Continue baking until casserole is bubbling at the edges, 5-10 minutes.

Check to see that casserole is done. Remove from oven or add time as needed.

Allow casserole to rest for 10-15 minutes before serving.