Preheat oven to 350°F and grease a 9x13 baking dish.
Boil spaghetti in salted water until al dente; drain and return to pot.
8 oz Spaghetti
In another pot, heat olive oil over medium heat. Add onion; cook until translucent. Add garlic; cook for 1 minute.
1 tbsp Olive oil, 1/2 cup Onion, 2 cloves Garlic
Melt butter with onions and garlic, then stir in flour. Cook for 1-2 minutes.
1/4 cup Butter, 1/4 cup All purpose Flour
Gradually whisk in chicken stock, then milk. Add nutmeg, salt, and pepper. Cook until it begins to boil and thickens.
1 1/2 cup Chicken stock, 1 cup Milk, 1 tsp Kosher salt, 1/2 tsp Black pepper, 1/4 tsp Nutmeg
Stir in 1 1/4 cups parmesan cheese. Remove from heat.
1 3/4 cup Parmesan cheese
Add cooked chicken and peas to the sauce; mix. Combine with cooked pasta.
2 cup Cooked chicken, 1/2 cup Frozen peas
Transfer to baking dish, top with remaining parmesan. Bake 25-30 minutes until golden.
Let rest 5-10 minutes before serving. Optional: garnish with parsley.