Preheat oven to 375°F (190°C) and prepare a 9x13" casserole dish with avocado oil cooking spray.
Cook the veggie spaghetti noodles until al dente according to the package instructions. Drain and set aside.
In a large skillet, heat the butter over medium heat. Add the chopped onions and bell pepper. Sauté until softened and fragrant for about 4-5 minutes.
Stir in the Italian seasoning, salt, pepper, and minced garlic and cook for another minute. Then, add in the flour.
Slowly pour in the chicken broth while stirring.
Add the shredded cheeses and cream cheese, and stir well until completely melted. Remove from heat.
Combine the diced tomatoes, cooked veggie noodles, and shredded chicken into the cheese sauce mixture. Toss until well combined.
Transfer the chicken spaghetti mixture to the lightly greased casserole dish. Sprinkle the grated shredded cheese evenly over the top. Garnish with a dash of the Italian seasoning or parsley.
Bake in the preheated oven for 25-30 minutes or until the cheese is melted and bubbly. Serve hot.