Green Bean Casserole With Crispy Fried Onions

Preheat oven to 425º. Line 2 rimmed baking sheets with parchment paper. Arrange 2 lb. mixed wild mushrooms, trimmed, halved or quartered if large (about ½" pieces) and 2 thyme sprigs in a single layer on prepared baking sheets. Drizzle with ¼ cup extra-virgin olive oil and season with season with ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt and ½ tsp. freshly ground black pepper. Roast mushrooms, stirring once and rotating pan halfway through, until lightly golden, crisp-tender, and liquid is evaporated (watch carefully as cook time can vary between types of mushrooms), 25–35 minutes. Remove sheets from oven and reduce oven temperature to 375ºF.

Meanwhile, generously season a large pot of boiling water with salt. Blanch 2 lb. green beans (preferably haricots verts), trimmed, cut in half on the diagonal, in salted boiling water until crisp-tender, 3–5 minutes. Drain in a colander and rinse under cold water to stop cooking. Drain well, then layer on a rimmed baking sheet with paper towels and pat dry. Wipe pot dry.

Melt 2 Tbsp. unsalted butter over medium-low heat in same pot. Add ⅓ cup finely chopped red onion and 2 large garlic cloves, very thinly sliced, and cook, stirring occasionally, until tender, 5–8 minutes. Stir in 2 Tbsp. all-purpose flour and cook, stirring occasionally, until aroma is mildly nutty, 4–6 minutes. Add 3 Tbsp. heavy cream and stir well to combine. Repeat twice, then stir in remaining cream, 1 cup low-sodium chicken broth or vegetable broth, 2 dried bay leaves, and remaining 2 thyme sprigs. Bring to a simmer and cook, stirring occasionally and scraping sides of pot, until sauce is the consistency of slightly thickened cream and coats the back of a wooden spoon, about 20 minutes (do not over-reduce). Discard thyme and bay leaves. Stir in 1½ tsp. finely grated lemon zest (from 1 medium lemon), ½ tsp. Diamond Crystal or ¼ tsp. + ⅛ tsp. Morton kosher salt, and ¼ tsp. freshly ground black pepper.

Arrange beans and mushrooms in 3-qt. baking dish. Pour cream sauce over; stir to evenly coat. Bake casserole until sauce is thickened and bubbling around the edges, about 20 minutes. Do ahead: Green beans, mushrooms, and sauce can be cooked 3 days ahead; refrigerate in separate airtight containers. Let come to room temperature before using.

Pour canola or vegetable oil into a large wide pot or high-sided skillet to a depth of ½". Place over medium-high heat until hot but not smoking (about 350° on a deep-fry thermometer). Line a rimmed baking sheet with paper towels.

Whisk ½ cup fine-grind cornmeal, 2 Tbsp. cornstarch, 1½ tsp. paprika, 1 tsp. ground coriander, and 1 tsp. ground cumin in a large shallow bowl. Toss 1 medium red onion, thinly sliced into ⅛" rings crosswise with a mandoline or very sharp knife, separated into rings in cornstarch mixture to coat.

Working in batches, fry onion rings, stirring and turning occasionally, until golden brown and crisp, 2–3 minutes. Using tongs or a slotted spoon, transfer to prepared sheet to drain. Season immediately with kosher salt and freshly ground black pepper.

Top green bean mixture with fried onions and serve warm. Do ahead: Onions can be fried 1 day ahead. Let cool. Arrange in a flat layer on a paper-towel-lined rimmed baking sheet, cover loosely with foil, and store at room temperature.