For the French fried onions:
Slice your onions as thinly as possible, using a Mandoline can help to get even and thin slices. Soak the onions in the beer for one hour, stirring to evenly coat the onions a couple of times. Preheat vegetable oil to 375 degrees F.
Mix the flour, salt and black pepper in a medium size mixing bowl. Dredge the onions in the flour mixture before putting them into the hot oil. Cook the onions for 1-2 minutes or until golden brown. Remove immediately and place on top of paper towels to help soak up any excess oil.
For the cream of mushroom reduction:
In a small bowl, whisk together cornstarch and 2 tablespoons cold water; set aside. This will help the cornstarch dissolve before it is added to the warm mushroom mix.
Melt butter in a large skillet over medium high heat. Add garlic, mushrooms, onion and thyme. Cook, stirring occasionally, until tender, about 3-4 minutes; season with salt and pepper, to taste. Stir in milk and half and half or heavy cream.
Bring to a boil; reduce heat and stir in cornstarch mixture until thickened, about 2-3 minutes. Keep warm on the stove.
For the casserole:
Preheat oven to 350 degrees F. Coat a 11x7-inch baking dish with nonstick spray.
In a large pot of boiling salted water, blanch green beans until bright green in color and tender crisp, about 2 minutes. Drain well and cool in a bowl of ice water. Drain well and set aside.
Turn the cream of mushroom reduction back to a simmer. Stir in green beans. Stir in 1 c. of the French fried onions.
Add green bean mixture to prepared baking dish. Sprinkle generously with fried onions. Place into oven and bake until bubbly and golden brown, about 30 to 35 minutes.