Green Bean Casserole with Onion Rings

1. Stage

Make Green Beans: Preheat oven to 350°. 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.

2. Stage

Meanwhile, in a large pot over medium-high heat, melt 2 tablespoons butter. Add onions and salt and cook, stirring often, until tender, about 5 minutes, then add mushrooms and salt and 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.

3. Stage

Melt remaining 5 tablespoons butter in same large pot, then add flour and cook, stirring constantly, until golden brown, about 2 minutes. Gradually whisk in milk and bring to a simmer, then cook until slightly thickened, about 4 minutes; season with salt. Pour sauce over green beans and mushrooms and toss to combine, then transfer to a 3-quart baking dish and cover with foil. Bake until warmed through and bubbling around the edges, about 20 minutes.

4. Stage

Make Onion Rings: Meanwhile, whisk cornstarch, salt, baking powder, baking soda, cayenne, and ½ cup flour in a large bowl, then add club soda and whisk until smooth. Place remaining ½ cup flour in a separate bowl, add onions, and toss to coat.

5. Stage

Fit a large heavy-bottomed pot with a deep-fry thermometer and heat oil to 350°. 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, then repeat with remaining onions.

6. Stage

Top warm green bean casserole with onion rings just before serving.