Preheat the oven to 450 degrees. Prepare a large baking sheet with parchment paper or aluminum foil spritzed with nonstick cooking spray.
In a medium bowl, stir together the flour, panko, and salt. Add the onions and toss to coat well.
Place the onions on the prepared baking sheet in a single layer. Bake for 30 minutes, stirring every 10 minutes. Decrease the oven temperature to 400 degrees.
Meanwhile, in a 12-inch skillet over medium-high heat, add the butter and the olive oil, melting the butter. Place the mushrooms in the skillet and cook, stirring often, until the mushrooms are tender, about 5 to 6 minutes.
Sprinkle the seasonings and flour over the mushrooms and stir to coat, cooking for about 1 minute. Slowly add the chicken broth, stirring to avoid clumps. After the chicken broth is fully incorporated, add half and half. Reduce the heat to low and cook until thickened, about 4 minutes.
Prepare a 9x13 casserole dish with nonstick cooking spray.
Add the green beans to the casserole dish.
Pour the sauce over the green beans and stir to incorporate. Bake the casserole, uncovered, for 20 to 30 minutes.
Add the crispy onions on top and return to the oven for an additional 5 minutes.