In a small bowl, mix together panko and fried onions with two tablespoons of olive oil. Set aside.
2
Fill a large bowl with ice water. Bring a large pot of water to a boil. of water to boil. Add two tablespoons of salt and the green beans. Cook beans until bright green and crisp-tender, approximately 6 minutes.
3
Drain beans in colander and plunge immediately into ice water to stop cooking. Set aside.
4
Add butter to your now-empty pot and melt over medium-high heat until foaming subsides. Add mushrooms, garlic, ¾ teaspoons salt, and ⅛ teaspoons pepper; cook until mushrooms release moisture and liquid evaporates, about 6 minutes.
5
Add flour and cook for 1 minute, stirring constantly. Stir in broth and bring to simmer, stirring constantly. Add cream, reduce heat to medium, and simmer until sauce is thickened and reduced to 3½ cups, about 12 minutes. Season with salt and pepper to taste.
6
Add green beans to sauce and stir until evenly coated. Arrange in even layer in 3 quart (or 13 x 9") baking dish.
7
Store your onion
8
Heat the casserole in a 425 degree oven for 10 minutes, then add the topping and bake for 1-20 minutes or until the top is golden brown and the sauce is bubbling on the edges.