Preheat the oven to 375°F (190°C). Grease a 9×13-inch baking dish and set aside.
Bring a large pot of salted water to a boil and cook the spaghetti until al dente according to package directions. Drain and set aside.
In a large skillet over medium-high heat, melt the butter. Add the minced garlic and chopped green bell pepper and sauté until the pepper is tender, about 3–4 minutes.
Sprinkle in the flour and Italian seasoning, stirring constantly and cooking for about 1 minute to remove the raw flour taste.
Slowly whisk in the heavy cream, chicken broth, and milk. Bring to a gentle boil, then reduce heat and simmer, stirring frequently, until the sauce thickens slightly.
Remove the skillet from the heat and stir in 1/2 cup of the shredded mozzarella until melted. Season the sauce with salt and pepper to taste.
Add the cooked spaghetti, shredded chicken, and drained diced tomatoes to the skillet and toss until evenly combined.
Transfer the mixture into the prepared baking dish and spread into an even layer. Sprinkle the remaining mozzarella evenly over the top.
Bake for 15–20 minutes, until the cheese is bubbly and the edges are lightly browned. Let sit a few minutes before serving.
