Preheat the oven to 350ºF.

Grease a 2½ quart casserole dish with cooking spray.

Bring a pot of salted water to a boil.

Add the green beans and cook until crisp-tender, about 4 minutes.

Drain and rinse with cold water.

Pat dry with paper towels.

Finely mince 1 shallot.

Place the bacon in a large skillet and cook over medium heat until the fat has been rendered and the bacon is crispy.

Remove from the pan and drain on paper towels.

Add the mushrooms to the pan and cook until they have released their liquid and started to brown, about 8 minutes.

Add the minced shallot and cook for 2-3 minutes or until the shallot is soft.

Sprinkle ¼ cup of flour over the mixture and cook, stirring, for 1 minute.

Stir in the milk and broth.

Simmer until the sauce has thickened, about 5 minutes.

Add the green beans and reserved bacon to the sauce.

Stir, season to taste with salt and pepper, and pour into the prepared baking dish.

Bake the casserole for 20 minutes or until the sauce is bubbly.

While the casserole bakes, heat 2 inches of oil in a heavy saucepan until it reaches 350ºF.

Cut the remaining shallots into 1/8-inch thick slices and separate into rings.

Coat the rings with the remaining 1/3 cup flour, shaking off the excess.

Fry in small batches until golden brown, about 30 seconds.

Place on paper towels to drain.

Top the baked casserole with the fried shallots and serve.