Ingredients:

  • 2 kg (approx. 2.6 lbs) Chicken Breast, cut into 1.5-inch cubes
  • 2 tbsp Unsalted Butter (for searing and sautéing)
  • 1 tbsp Olive Oil
  • 2 medium Leeks, white and light green parts only, sliced thinly
  • 8 oz White or Cremini Mushrooms, sliced
  • 3 cloves Garlic, minced
  • 1 tsp Dried Thyme
  • Salt and Black Pepper, to taste
  • 4 tbsp Unsalted Butter (for roux)
  • 4 tbsp All-Purpose Flour
  • 2 cups Chicken Stock (low sodium), warmed
  • 1 cup Whole Milk, warmed
  • 1/2 cup Double Cream (Heavy Cream)
  • 1 tsp Dijon Mustard
  • Pinch Nutmeg, freshly grated
  • 2 slices Day-old Bread, processed into coarse crumbs
  • 1/2 cup Grated Parmesan Cheese (or mature cheddar)
  • 2 tbsp Melted Butter (to bind the crumbs)
  • 2 tbsp Fresh Parsley, chopped

Instructions:

  1. Sear the Chicken: Preheat oven to 200°C / 400°F. Pat chicken cubes dry and season well. Heat olive oil and 1 tbsp butter in a large skillet. Sear chicken in batches until lightly golden on the outside (about 2 minutes per side). Remove and set aside.
  2. Sauté Aromatics: Reduce heat to medium. Add the remaining 1 tbsp of butter. Add sliced leeks and mushrooms to the skillet. Cook gently for 8-10 minutes, stirring occasionally, until softened and the mushrooms have released their moisture.
  3. Add Garlic and Thyme: Stir in the minced garlic and dried thyme. Cook for 1 minute until fragrant. Remove the vegetable mixture and combine with the reserved chicken in a large bowl.
  4. Make the Roux: Return the empty skillet to medium heat. Melt the 4 tbsp of butter. Whisk in the flour quickly to form a thick paste (the roux). Cook the roux for 2 minutes, stirring constantly, until nutty.
  5. Build the Béchamel: Slowly pour in the warm chicken stock, whisking continuously to prevent lumps. Once smooth, slowly incorporate the warm milk. Bring to a gentle simmer, stirring until the sauce thickens significantly (it should coat the back of a spoon).
  6. Finish the Sauce: Remove from heat. Stir in the heavy cream, Dijon mustard, nutmeg, and season with salt and pepper.
  7. Combine Filling and Assemble: Pour the finished sauce over the chicken and vegetable mixture. Stir gently until evenly coated. Transfer the filling into a prepared 9x13 inch baking dish.
  8. Prepare Topping and Bake: In a small bowl, combine the breadcrumbs, grated Parmesan cheese, and melted butter. Toss until coated. Sprinkle the crumb mixture evenly over the casserole. Bake for 30–35 minutes, or until bubbling hot and the topping is deeply golden brown.
  9. Rest and Serve: Allow the casserole to rest on the counter for 10 minutes before serving. Garnish with fresh parsley.