Preheat oven to 375 degrees.Add 1 tablespoon olive oil to a large skillet. Add in green beans and sauté for 3-4 minutes. Set aside. (They will still be crunchy that is fine they will continue to cook in the oven.)
Crispy Onion Topping
Add 1/2 tablespoon of butter to the same skillet as the green beans. Add onion and sauté for 2-3 minutes or until slightly tender (they continue to cook in the oven). Set aside in a medium bowl.
Add the remaining 1/2 tablespoon of butter to the same skillet. Add panko and cook until golden brown and toasted, 1-2 minutes. Remove from the pan and toss with the onions along with the parmesan cheese, salt and pepper. Set aside.
Mushroom Cream Sauce
Heat butter in a medium sauce pan. Add mushrooms and saute for 3-4 minutes or until tender and golden brown.
Add garlic and sauté for 1 additional minute. Whisk in flour and stir constantly for 1 minute.
Slowly pour in chicken stock and milk. Allow the sauce to simmer stirring constantly until thickened. Season with salt and pepper to taste.
Assembly
Toss the green beans in the mushroom sauce and spread in a 9 x 13 baking dish.
Top with the crispy onion panko topping. Bake for 20-25 minutes or until golden brown and warmed through.