Ingredients:

  • 4 large shallots (or 1 small white onion), thinly sliced
  • 3 cups vegetable oil or peanut oil (700 ml), for frying
  • 1 Tbsp all-purpose flour (15 g)
  • Pinch of fine sea salt
  • 2 lbs fresh French green beans (haricots verts), trimmed (900 g)
  • 1 tsp kosher salt (5 g)
  • 2 slices thick-cut bacon or pancetta, diced (50 g)
  • 1 Tbsp unsalted butter (15 g)
  • 8 oz cremini (baby bella) mushrooms, sliced (225 g)
  • 2 cloves garlic, minced
  • 1/4 cup all-purpose flour (30 g)
  • 1 cup full-fat whole milk, warmed (240 ml)
  • 1 cup low-sodium chicken stock (240 ml)
  • 1/2 cup heavy cream (120 ml)
  • 1 tsp Dijon mustard
  • 1 Tbsp Worcestershire sauce
  • 1/2 tsp freshly grated nutmeg
  • 1/2 tsp black pepper
  • 1 tsp kosher salt (or to taste)
  • 1/2 cup grated sharp cheddar or Gruyère cheese (optional)

Instructions:

  1. Prepare the Topping (Crispy Shallots): Add the vegetable oil to a deep saucepan and heat to 325°F (160°C). Toss the thinly sliced shallots with 1 Tbsp of flour and a pinch of salt.
  2. Fry the Shallots: Carefully drop the floured shallots into the hot oil in small batches. Fry for 3–5 minutes, stirring occasionally, until golden brown and crisp. Remove immediately with a slotted spoon and spread thinly on a wire rack lined with paper towels. Season immediately with salt and set aside.
  3. Blanch the Green Beans: Bring a large pot of salted water to a rolling boil. Add the trimmed green beans and cook for 3–4 minutes until they are bright green and tender-crisp (al dente). Immediately drain the beans and plunge them into an ice bath to stop the cooking process. Drain well and set aside.
  4. Start the Sauce Base: Preheat the oven to 375°F (190°C). In a large saucepan, cook the diced bacon until crisp. Remove the bacon bits (reserve for later), leaving the fat behind. Add the 1 Tbsp butter and the sliced mushrooms to the fat. Sauté until the mushrooms are deeply browned (about 6–8 minutes). Add the minced garlic and cook for 1 minute.
  5. Make the Roux: Sprinkle the 1/4 cup flour over the mushroom mixture. Stir constantly for 2 minutes to cook out the raw flour taste, forming a roux.
  6. Thicken the Sauce: Gradually and slowly whisk in the warm milk and chicken stock until smooth and lump-free. Bring the mixture to a simmer, stirring continuously until the sauce thickens considerably.
  7. Finish the Sauce: Stir in the heavy cream, Dijon mustard, Worcestershire sauce, nutmeg, black pepper, and salt. If using, stir in the grated cheese until melted and smooth. Remove from heat.
  8. Assemble the Casserole: Fold the blanched green beans and the reserved crispy bacon bits into the creamy mushroom sauce. Taste and adjust seasoning. Pour the mixture into a prepared 9x13-inch baking dish.
  9. Bake: Bake uncovered for 20–25 minutes, or until the casserole is bubbling hot throughout.
  10. Serve: Remove from the oven. Gently top the casserole with the prepared crispy shallots. Return to the oven for 3–5 minutes only to warm the topping slightly (do not let them burn or get soggy). Serve piping hot immediately.