- 2¼ pounds / 1 kilogram green beans, trimmed and split lengthwise
- 1 cup / 225 grams unsalted butter, sliced
- 3 medium to large sweet onions, thinly sliced (about 4½ cups)
- 2 pounds / 908 grams cremini mushrooms, quartered
- 1½ teaspoons / 9 grams fine sea salt, divided, plus extra for the blanching water and seasoning
- ¼ cup / 37 grams all-purpose flour
- 1½ cups / 250 grams reserved onion broth or vegetable stock (from Pearl Onions and
- Grapes, page 146)
- 12½ ounces / 340 grams shredded white cheddar cheese (6½ cups)
- 1 cup / 15 grams dried shiitake mushrooms
- 14 tablespoons / 310 grams Clarified Butter, divided (page 57)
- 2 shallots, sliced 1/8-inch thick
- 1 cup / 225 clarified butter (page 57)
- 9 ounces / 260 grams fresh shiitake mushrooms, stems removed, thinly sliced
Set a large pot of water over high heat. Season with salt (I like to use 20 grams [¾ ounce] of salt for every quart of water), and bring to a boil. Prepare an ice bath. Blanch the green beans in the boiling water for 30 seconds. You will need to do this in batches so you don't crowd the pot. Immediately use a skimmer to transfer the beans to the ice bath to cool. Drain the green beans and reserve.
Put ½ cup (113 grams) of the butter in a large skillet that has a lid and set over low heat. Once the butter melts, add the onions and mix them thoroughly to coat the onions with the butter. Cover and cook undisturbed for 30 minutes. You don't want the onions to take on any color.
Meanwhile, cook the cremini mushrooms. Set a large sauté pan over medium-high heat. Put the remaining butter in the sauté pan and let it melt. Add the mushrooms, season with ¾ teaspoon (4.5 grams) salt, and cook, stirring, until they turn a deep golden brown. Use a slotted spoon to transfer to a plate lined with paper towels to drain and cool. Pour any remaining butter from the sauté pan into the pan with the onions and mix it in.
After 30 minutes, season the onions with the remaining ¾ teaspoon (4.5 grams) salt and continue to cook for another 30 minutes, stirring every 8 to 10 minutes so they don't start to stick to the bottom of the pan. At the end of 1 hour, there still should be no color on the onions or the bottom of the pan. Add the flour and lightly stir to incorporate. The mixture will start to form a lumpy mass in irregular patterns throughout the pan. Add the onion broth and stir to incorporate completely. Cook the mixture, stirring gently, for 6 minutes, or until the raw flour taste is gone. Remove the pan from the heat and transfer the onions to a blender. Blend on high speed until the mixture is smooth. Pour into a clean saucepot, set over medium heat, and bring to a simmer. Whisk in the cheddar cheese a little bit at a time until it is totally incorporated and the mixture has thickened.
Preheat the oven to 350°F (180°C).
Arrange the green beans in a large baking dish or casserole. Sprinkle the cremini mushrooms throughout the beans. Pour the cheese sauce over the green beans and mix gently to thoroughly coat the vegetables. Cover with aluminum foil or a lid and bake for 35 minutes.
While the casserole is baking, put 1 cup (225 grams) of the Clarified Butter in a medium pot and set over medium-high heat. Add the sliced shiitake mushrooms and cook, stirring, until they become golden brown and crisp. Use a slotted spoon to transfer the mushrooms to a plate lined with paper towels to drain and cool. Once cooled and crisp, put the mushrooms in a blender or spice grinder and pulverize.
Put the shallots in a medium pot and pour the clarified butter over them. Stir the shallots and set the pot over medium-low heat. Cook, stirring occasionally, until the shallots begin to fry and brown, about 10-15 minutes. Once the shallots are a deep golden brown, use a slotted spoon to transfer them to a plate lined with paper towels. Season them lightly with salt and let cool. Strain the butter into a clean pot and add the mushrooms. Set the pot over medium-low heat and cook, stirring occasionally, until the mushrooms begin to fry and brown, about 10-15 minutes. Once the mushrooms are crisp and brown, use a slotted spoon to transfer them to a plate lined with paper towels. Season lightly with salt and let cool. Once the shallots and mushrooms are completely cool, mix them together.
After 35 minutes, take the green beans out of the oven and uncover them. Sprinkle the fried shallots and mushrooms evenly over the top and bake, uncovered, for 20 minutes, or until they are golden brown.