Ingredients:

  • Tbsp Unsalted Butter (for sautéing)
  • Tbsp Olive Oil
  • large Yellow Onion, finely diced
  • oz Cremini Mushrooms, sliced
  • cloves Garlic, minced
  • /4 cup Dry White Wine (e.g., Pinot Grigio)
  • Tbsp All-Purpose Flour
  • cups Chicken Stock (low sodium)
  • cup Whole Milk or Half-and-Half
  • tsp Fresh Thyme leaves
  • /2 tsp Dijon Mustard
  • cups Cooked Chicken Breast or Thighs, shredded or diced
  • cup Frozen Peas
  • cup Gruyère Cheese, grated
  • /2 cup Parmesan Cheese, grated
  • Salt and freshly ground Black Pepper, to taste
  • Tbsp Unsalted Butter, melted (for topping)
  • large Leek, white and light green parts only, thinly sliced and thoroughly washed
  • /2 cup Panko Breadcrumbs
  • /4 cup Dry Breadcrumbs (standard)
  • Tbsp Fresh Parsley, chopped

Instructions:

  1. Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
  2. Melt 2 Tbsp butter and oil in a large oven-safe skillet over medium heat. Sauté onions until softened (about 5 minutes). Add mushrooms and cook until browned and moisture has evaporated (another 5–7 minutes). Add garlic and thyme; cook 1 minute until fragrant.
  3. Pour in the white wine, scraping up any browned bits from the bottom of the pan. Let it bubble and reduce by half.
  4. Sprinkle the flour over the mixture and stir constantly for 1 minute to cook out the raw flour taste (the roux).
  5. Gradually whisk in the chicken stock, ensuring no lumps form. Once smooth, whisk in the milk. Bring to a gentle simmer, stirring often, until the sauce thickens enough to coat the back of a spoon. Remove from heat.
  6. Stir in Dijon mustard, shredded chicken, peas, 3/4 of the Gruyère, and half of the Parmesan. Season aggressively with salt and pepper. Taste test!
  7. Pour the filling mixture into the prepared baking dish.
  8. In a separate small bowl, combine the 2 Tbsp melted butter with the washed, sliced leeks. In another bowl, mix the Panko, standard breadcrumbs, and parsley.
  9. Scatter the leek mixture evenly over the casserole filling. Then, scatter the remaining Gruyère and Parmesan over the top.
  10. Bake for 35–40 minutes, or until the filling is bubbly throughout and the topping is deep golden brown and crispy. Loosely tent with foil if the topping browns too quickly.
  11. Let the casserole rest for 10 minutes before serving to allow the sauce to set.