For the topping:
2 medium onions, thinly sliced
1/4 cup all-purpose flour
2 tablespoons panko breadcrumbs
1 teaspoon kosher salt
Non-stick spray

For beans and sauce:
1 pound fresh green beans, rinsed, trimmed and halved
2 tablespoons unsalted butter
12 ounces mushrooms, trimmed and cut into 1/2 inch pieces
1 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
2 cloves garlic, minced
1/4 teaspoon freshly ground nutmeg
2 tablespoons all-purpose flour
1 cup chicken broth
1 cup half and half