Preheat oven to 200°C (400°F). Lightly grease a 23x33cm (9×13 in) baking dish.
Cook the egg noodles according to package directions. Drain and rinse with cold water. Set aside.
In a saucepan, melt the butter over medium heat. Add the onion and sauté for 5-7 minutes until tender.
Sprinkle the flour and garlic over the onions. Cook for 1-2 minutes stirring frequently.
Gradually whisk in the chicken broth and milk. Season with salt and pepper. Bring to a simmer and cook for 2-3 minutes until thickened.
Stir in the frozen peas, cheddar cheese and cooked chicken. Remove from heat and stir until cheese melts.
Add the cooked egg noodles to the prepared baking dish. Pour the chicken mixture over the noodles and toss to coat evenly.
In a small bowl, mix together the crushed Ritz crackers, Parmesan cheese and olive oil. Sprinkle over the casserole.
Bake for 15-20 minutes until the cracker topping is lightly browned.
Garnish with parsley if desired. Serve warm.
