Preheat your oven to 190°C (375°F). Lightly grease a 23cm x 33cm (9x13 inch) baking dish with butter or cooking spray.

Cook the egg noodles according to package directions until al dente. Be careful not to overcook them, as they will continue to soften in the oven. Drain the noodles thoroughly and set aside.

While the noodles are cooking, prepare the sauce base. Melt 60g (4 tbsp) of butter in a large saucepan or Dutch oven over medium heat. Add the finely chopped 1 medium yellow onion (approx. 150g / 5.3 oz), 2 finely chopped celery stalks (approx. 80g / 2.8 oz), and 2 finely chopped medium carrots (approx. 120g / 4.2 oz). Sauté the vegetables, stirring occasionally, until they have softened, about 8-10 minutes. Avoid browning them; you want them tender.

Add the 3 minced garlic cloves to the saucepan with the softened vegetables. Cook for another minute until fragrant, stirring constantly. Be cautious not to burn the garlic.

Sprinkle the 60g (1/2 cup) of all-purpose flour over the vegetables. Stir well to coat the vegetables and cook for 1-2 minutes, stirring constantly. This step cooks out the raw flour taste and helps thicken the sauce.

Gradually whisk in the 500ml (2 cups) of chicken broth, ensuring no lumps of flour form. Once the broth is incorporated, slowly whisk in the 250ml (1 cup) of milk. Continue to stir until the sauce begins to thicken.

Stir in the 1 tsp garam masala, 1 tsp dried tarragon, 1 tsp salt, and 1/2 tsp black pepper. Taste the sauce and adjust seasoning as needed. Remember that the turkey and cheese will add saltiness.

Add the 500g (1 lb) of cooked turkey or chicken (shredded or diced) and 150g (1 cup) of frozen peas to the sauce. Stir gently to combine and allow the mixture to heat through for about 2-3 minutes. The peas should be tender-crisp.

Gently fold the drained cooked egg noodles into the creamy turkey mixture. Ensure all the noodles are evenly coated with the sauce.

Pour the noodle and turkey mixture into the prepared baking dish, spreading it evenly.

In a small bowl, combine the 100g (1 cup) of shredded cheddar cheese and 50g (1/2 cup) of breadcrumbs. Sprinkle this mixture evenly over the top of the casserole.

Bake in the preheated oven at 190°C (375°F) for 20-25 minutes, or until the topping is golden brown and the casserole is bubbly around the edges. If the topping starts to brown too quickly, you can loosely tent the dish with aluminum foil.

Let the casserole rest for about 5-10 minutes before serving. This allows the sauce to set slightly, making it easier to serve.

Garnish with chopped fresh parsley, if desired, before serving.