Preheat the oven to 350°F. Spray a 9x13-inch baking dish with cooking spray, set aside.
Cook the spaghetti noodles per the directions on the back of the box, drain well, and set aside.
In a large pot over medium heat, add the butter. Once melted, add the onion and cook until softened, stirring occasionally, for 10 minutes.
Add the garlic and stir it in until fragrant (about 30 seconds.)
Stir in the flour and cook for 1 minute.
Slowly stream in the milk and chicken stock while constantly whisking to avoid lumps.
Add the cream of mushroom soup and whisk it in until combined.
Add the chicken, peas, sour cream, dried parsley, salt, and pepper, and stir well until everything is combined.
Lastly, add the cooked noodles to the pot and coat the noodles well in the sauce.
Pour everything into the prepared baking dish. Top evenly with the shredded mozzarella cheese.
Bake uncovered for 30-35 minutes until bubbly. If desired, turn the broiler on high and broil the top to lightly brown the cheese. Keep an eye on it, or it might burn.
Garnish with more dried parsley, if desired, and serve!
Video
Notes
Other pasta can be used, see above on options.
Add other frozen vegetables to your liking.
Switch out the cheese, see some of my favorites above.
This can be frozen, see above on how to do that.
To make this richer you can use half and half or heavy cream instead of milk.
Use your favorite shredded chicken or even diced.
Turkey and ham will work with this recipe.
Other soups can be used, see above for some suggestions.