Preheat oven to 350 degrees F.
In a large skillet, heat the 2 tablespoons olive oil. Add shallots and garlic, saute' for a couple of minutes until soft. Add all of the mushrooms and saute' until cooked and lightly browned. Add the white wine, bring to a boil and reduce by half. Remove from skillet and set aside.
In the same skillet add butter and flour, cook stirring often, over med/low heat until very light golden brown. Slowly whisk in the half and half and bring to a boil then reduce heat to low. Add mushroom mixture and season with salt and pepper.
In a large casserole dish, add the thawed green beans and 1/2 of the crispy onions (3/4 cup). Pour mushroom/cream mixture over the top and stir to evenly coat.
Cover with foil and bake in the preheated oven for 30 minutes.
Remove foil and top the casserole with the rest of the crispy onions and grated cheese. Place back in the oven and bake uncovered for 10-15 minutes until crisped and bubbly. Serve.