Preheat the oven to 375 degrees. Coat a large baking dish with cooking spray.
Blanch the green beans in a large pot of boiling water for 3-4 minutes or until just tender. Strain the green beans and rinse them under cold water to stop them from cooking.
Heat 2 tablespoons of butter in a large skillet over medium-high heat. Once the pan is hot, add the mushrooms and cook, without stirring for 2-3 minutes. Flip the mushrooms and continue to cook for 2-3 minutes or until golden brown.
Add the shallot to the pan and sauté for 1 minute, stirring often. Add the minced garlic to the skillet and cook, stirring constantly for 1 minute. Season with a bit of sea salt, to taste. Remove the mushroom mixture from the skillet to a bowl and set it aside.
Place the skillet back on the stove and add the remaining 2 tablespoons of butter, once it has melted, add the flour and whisk for 1-2 minutes. Slowly add the milk and whisk until thick and creamy without any lumps. Season with sea salt and freshly cracked pepper, to taste.
Add the green beans and mushrooms to the mixture and toss to coat evenly. Place the green bean mixture into the large baking dish and cover with a lid or foil.
Place into the oven to bake for 20 minutes.
Remove from the oven and sprinkle generously with French's fried onions. Return to the oven and bake for an additional 2-3 minutes or until the onions are golden brown. Be careful--the onions burn quickly.