Preheat the oven to 350℉.
In a large salted pot of water, cook the spaghetti according to package instructions. Place in a deep 9x13 inch casserole dish.
Meanwhile, in a medium saucepan, melt the butter. Mix in the flour until combined. Add in the chicken broth and milk, and mix until fully incorporated. Bring to a boil, then mix in the Parmesan cheese until melted. Remove from heat.
In a large saucepan, heat the oil. Sauté the onion for around 2 minutes, then add the green peas and turkey. Cook until warm.
Top the spaghetti with the turkey mixture, then the cheese sauce. Using tongs, mix the ingredients until the spaghetti is fully coated in sauce.
Place in the preheated oven for around 1 hour, or until the casserole is bubbling.