Preheat oven to 375°F (190°C) and lightly coat a 9x13-inch casserole dish with avocado oil spray.
Cook the veggie spaghetti noodles according to the package directions until al dente. Drain well and set aside.
In a large skillet, melt the butter over medium heat. Add the chopped onions and bell pepper and sauté for about 4–5 minutes, until soft and fragrant.
Stir in the Italian seasoning, salt, pepper, and minced garlic, cooking for another minute. Then, sprinkle in the flour and stir to combine. It’ll help thicken your sauce later.
Gradually pour in the chicken broth, whisking constantly to create a smooth sauce base.
Add the shredded cheeses and cream cheese, stirring until everything is fully melted and creamy. Remove the skillet from heat.
Combine then the diced tomatoes, cooked veggie noodles, and shredded chicken. Toss gently until everything is evenly coated in the cheesy sauce.
Pour the chicken spaghetti mixture into the prepared casserole dish. Sprinkle the remaining shredded cheese evenly on top, and finish with a dash of Italian seasoning or chopped parsley for color.
Bake for 25–30 minutes, or until the top is melted, golden, and bubbly.