Green Bean Casserole with Onion Rings

Make Green Beans: Preheat oven to 350 degrees F. Cook green beans in a large pot of boiling salted water until bright green, 6 to 8 minutes. Transfer to an ice bath to cool, then drain. Meanwhile, melt 2 tablespoons butter in a large pot over medium-high heat. Add onions and 1 teaspoon salt and cook, stirring often, until tender, about 5 minutes. Add mushrooms, 1/2 teaspoon salt, and 1/4 teaspoon pepper and cook, stirring often, until brown and tender, 5 minutes more. Transfer onions and mushrooms to a large bowl, add green beans, and toss to combine. Melt remaining 5 tablespoons butter in same large pot. Add flour and cook, stirring constantly, until golden brown, about 2 minutes. Gradually whisk in milk and bring to a simmer. Cook until slightly thickened, about 4 minutes; season with salt. Pour sauce over green beans and mushrooms and toss to combine. Transfer to a 3-quart baking dish and cover with foil. Bake until warmed through and bubbling around the edges, about 20 minutes. Make Onion Rings: Meanwhile, whisk cornstarch, salt, baking powder, baking soda, cayenne, and 1/2 cup flour in a large bowl. Add club soda and whisk until smooth. Place remaining 1/2 cup flour in a separate bowl, add onions, and toss to coat. Fit a large heavy-bottomed pot with a deep-fry thermometer and heat oil to 350 degrees F. Working in batches, dip onion rings into batter and carefully lower into oil, being careful not to overcrowd the pot. Fry, turning occasionally, until golden brown, 2 to 3 minutes. Transfer to a wire rack set on a baking sheet and season with salt. Repeat with remaining onions. Top warm green bean casserole with onion rings just before serving.