Step 1: Saut the Vegetables: Heat olive oil in a large oven-safe pot or Dutch oven over medium heat. Add chopped onion, carrots, and celery. Saut for 5-7 minutes, or until softened, stirring occasionally to prevent burning.
Step 2: Cook the Chicken: Add the chicken pieces to the pot and cook until browned on all sides and cooked through, about 5-7 minutes. Stir occasionally to ensure even cooking.
Step 3: Add Noodles and Broth: Stir in the whole wheat egg noodles and chicken broth. Bring to a simmer, then reduce heat to low, cover, and cook for 10-12 minutes, or until the noodles are tender. Be sure to stir occasionally to prevent noodles from sticking to the bottom of the pot.
Step 4: Prepare the Sauce: In a small bowl, whisk together the milk and flour until smooth. This will create a slurry to thicken the sauce.
Step 5: Thicken the Casserole: Pour the milk-flour mixture into the pot with the chicken and noodles. Stir well to combine and cook for another 2-3 minutes, or until the sauce has thickened slightly.
Step 6: Add Peas and Season: Stir in the frozen peas, dried thyme, salt, and pepper. Taste and adjust seasoning as needed.
Step 7: Bake (Optional): If the pot is oven-safe, transfer it directly to the preheated oven. Otherwise, pour the casserole into a greased 9x13 inch baking dish. Bake uncovered at 375F (190C) for 15-20 minutes, or until bubbly and slightly browned on top. This step enhances flavor, but is not essential.
Step 8: Garnish and Serve: Remove from oven (if baked) and let stand for a few minutes before serving. Garnish with chopped fresh parsley, if desired. Serve hot.
Optional Tips: For a creamier casserole, add 1/4 cup of plain Greek yogurt during the last few minutes of cooking (do not boil). For a gluten-free version, use gluten-free noodles and a cornstarch slurry instead of flour. Different vegetables (broccoli, green beans, bell peppers) can be added or substituted according to preference.