Ingredients
Scale
- 2 pounds (900 g) fresh green beans, trimmed and cut in half
- 1 tablespoon fine salt, for the blanching water
- A large bowl of ice water
- 2 large yellow onions (about 1 pound / 450 g), halved and sliced very thin
- 1/2 cup (120 ml) buttermilk
- 3/4 cup (95 g) all-purpose flour, for the onion coating
- 1 teaspoon fine salt, for the onion coating
- 1/2 teaspoon black pepper, for the onion coating
- 1/2 teaspoon smoked paprika, for the onion coating
- 3 tablespoons neutral oil, for the sheet pans
- 4 tablespoons unsalted butter
- 12 ounces (340 g) cremini mushrooms, sliced
- 2 cloves garlic, minced
- 1 teaspoon fresh thyme leaves (or 1/2 teaspoon dried thyme)
- 3 tablespoons all-purpose flour, for the sauce
- 1 1/2 cups (360 ml) low sodium chicken or vegetable stock
- 1 cup (240 ml) whole milk
- 1/2 cup (120 ml) heavy cream
- 1 teaspoon Dijon mustard
- 1/4 teaspoon ground nutmeg
- 3/4 teaspoon fine salt, plus more to taste
- 1/2 teaspoon black pepper
- 1/2 cup (50 g) finely grated Parmesan
Instructions
- Bring a large pot of water to a rolling boil and add the tablespoon of salt. Blanch the green beans for 4 minutes, until bright green and just tender. Lift them straight into the ice water, leave them 5 minutes, then drain very well.
- Spread the beans on a clean kitchen towel and pat them dry, then let them air dry while you work. This step decides whether your sauce stays thick, so do not rush it.
- Heat the oven to 425°F (220°C). Toss the onion slices with the buttermilk and let them sit for 10 minutes. Mix the flour, salt, pepper and paprika in a wide bowl, then lift the onions out of the buttermilk, let the excess drip off, and toss them in the flour until every strand is coated.
- Oil two large sheet pans generously. Spread the onions in loose single layers, drizzle with a little more oil, and bake for 18 to 22 minutes, tossing once halfway, until deep golden and crisp. Set aside; they crisp further as they cool.
- Lower the oven to 375°F (190°C). Melt the butter in a large, deep skillet over medium high heat. Add the mushrooms in a single layer and cook, undisturbed for the first 3 minutes, then stirring now and then, for 8 to 10 minutes total, until their liquid has evaporated and the edges are well browned.
- Stir in the garlic and thyme and cook for 30 seconds. Sprinkle the flour over the mushrooms and stir for 1 full minute, scraping the bottom of the pan.
- Pour in the stock a splash at a time, whisking after each addition, then add the milk and cream. Simmer gently for 4 to 5 minutes, stirring often, until the sauce thickly coats the back of a spoon.
- Take the pan off the heat. Whisk in the Dijon, nutmeg, salt and pepper, then stir in the Parmesan until it melts. Taste and adjust the salt: the beans will dilute the seasoning a little.
- Fold the dry green beans through the sauce until every bean is coated, then scrape everything into a 9×13 inch baking dish (or a 2 1/2 quart oval dish) and level the top.
- Bake for 18 to 20 minutes, until the sauce bubbles at the edges and in a few spots in the center. Scatter the crispy onions over the whole surface and return the dish to the oven for 4 to 5 minutes, just to heat the onions through.
- Rest the casserole for 10 minutes before serving. The sauce sets slightly, scoops come out clean, and nobody burns their tongue on the first bite.
- Prep Time: 25 minutes
- Cook Time: 45 minutes
Nutrition
- Serving Size: 1
- Calories: 245
- Sugar: 6
- Sodium: 480
- Fat: 16
- Saturated Fat: 8
- Unsaturated Fat: 7
- Trans Fat: 0
- Carbohydrates: 19
- Fiber: 4
- Protein: 7
- Cholesterol: 35