6 cups green beans, trimmed and cut into 1-inch pieces (about 1 ½ pounds)
4 ½ teaspoons extra-virgin olive oil, divided
1 tablespoon unsalted butter
3 shallots, minced
8-ounces Crimini or white mushrooms, finely chopped
2 teaspoons chopped fresh thyme
½ teaspoon salt
¼ teaspoon pepper
¼ cup dry sherry
1/3 cup all-purpose flour
1 cup reduced-sodium chicken broth or vegetable broth
1 cup whole milk
2 cups sharp cheddar, shredded and divided, 8-ounces
3 tablespoons dry breadcrumbs
½ teaspoon paprika
¼ teaspoon onion powder
Preheat oven to 425 degrees F. Coat a 2 1/2 quart baking dish with cooking spray.
Meanwhile, bring several inches of water to a boil in a large saucepan fitted with a steamer basket. Steam green beans until crisp tender, about 4 minutes. Carefully remove steamer from the saucepan, to stop the beans from cooking further.
Heat 4 teaspoons oil and butter in a large skillet over medium-high heat. Add shallots and cook until just starting to brown, 2 to 3 minutes. Add mushrooms, thyme, salt and pepper and cook, stirring occasionally until the mushrooms have released their juices and the liquid has evaporated, 3 to 4 minutes. Add sherry and cook, stirring occasionally until the liquid has evaporated, 2 to 3 minutes.
Sprinkle flour over the mushroom mixture and stir to coat. Add broth and milk and bring to a simmer stirring often until the sauce is thickened, about 5 minutes. Remove from the heat and stir in 1 ½ cups (6-ounces) cheddar. Add the green beans and stir to combine. Transfer to the prepared casserole dish. Top with the remaining 1/2 cups (2-ounces) cheddar.
Stir together the breadcrumbs and the remaining ½ teaspoon oil in a small dish. Add paprika and onion powder and stir until the breadcrumbs are evenly moist and bright orange. Sprinkle over the casserole.
Transfer the casserole to the oven and bake until bubbling and golden brown on top, 20 to 25 minutes. Allow to cool 15 minutes before serving.