Turn a classic holiday side dish gluten-free with a healthy version of cream of mushroom soup and a homemade crispy onion topping. No one will even know it's wheat-free!
Preheat oven to 400 degrees F. Coat a rimmed baking sheet with cooking spray. Coat a 9-by-13-inch baking dish with cooking spray.
Cut onion in half; thinly slice one half and dice the other half. Combine the sliced onion in a small bowl with buttermilk; let stand for 10 minutes. (Set the diced onion aside.)
Drain the sliced onion (discard the buttermilk). Add gluten-free flour and 1/4 teaspoon pepper to the onion and toss to coat. Spread out on the prepared baking sheet and coat with cooking spray. Bake for 10 minutes. Stir and continue baking until the onion slices are golden brown, about 10 minutes more.
Meanwhile, bring 1 inch of water to a boil in a large pot fitted with a steamer basket. Add beans, cover and steam until tender-crisp, 3 to 4 minutes. Remove the beans and basket from the pot.
Wipe the pot dry. Add oil and heat over medium heat. Add the diced onion and cook, stirring often, until softened, about 4 minutes. Stir in mushrooms, garlic, salt, thyme and the remaining 3/4 teaspoon pepper; cook, stirring often, until the mushrooms are tender and the mushroom liquid is almost evaporated, 3 to 5 minutes.
Pour in milk and bring to a simmer. Combine water and cornstarch in a small bowl. When the milk is simmering, stir in the cornstarch mixture and cook, stirring, until thick enough to coat the back of a spoon, 1 to 2 minutes. Stir in the green beans. Transfer the mixture to the prepared baking dish. Top with the crispy onion slices.
Bake the casserole until bubbling, 20 to 30 minutes.
1 1/2 vegetable, 1/2 fat