Place the green beans in a steamer pot over boiling water and steam for 8 minutes or until fork tender.

Preheat oven to 400°F.

In a large sauce pot melt 1 tablespoon butter and sauté the chopped onion and 1/4 teaspoon of salt for 4 minutes.

Remove the onions and set aside in a separate bowl with the panko and parmesan cheese. Toss to evenly combine.

In the same large sauce pot, melt the remaining 2 tablespoons of butter, add the mushrooms and remaining 1 teaspoon of salt and sauté for 4 minutes.

Add the minced garlic and cook for an additional 1 minute.

Sprinkle 2 tablespoons of flour over the mushrooms and stir for 30 seconds until evenly coated.

Add the chicken broth and milk, bring to a simmer and cook for 8-10 minutes or until thickened. If you run your spoon through the sauce and it immediately runs back together, continue to reduce.

Add the beans to the sauce and then pour into a greased 8 x 8 inch baking dish.

Sprinkle with onion/panko mixture on top, lightly spray with cooking oil and bake for 25 minutes or until the top is golden.