Start by bringing a large pot of salted water to a boil. Add the dry spaghetti noodles and cook according to package instructions until al dente. Drain and set aside.
In a large skillet over medium heat, melt the unsalted butter. Add the minced garlic and chopped green bell pepper, cooking until the garlic is fragrant and the bell pepper is tender, about 3-5 minutes.
Sprinkle the all-purpose flour over the sautéed vegetables and stir well to combine, cooking for another minute. Gradually whisk in the heavy whipping cream, chicken broth, and milk. Continue to stir until the mixture is smooth and begins to thicken.
Stir in the Italian seasoning, salt, and pepper to taste. Let the sauce simmer for a few more minutes to develop the flavors.
Add the shredded cooked chicken and drained diced tomatoes to the sauce, mixing well. Finally, fold in the cooked spaghetti noodles, ensuring everything is well-coated in that creamy goodness.
Top the spaghetti with shredded mozzarella cheese, covering the entire dish. You can either let it melt on the stovetop over low heat or transfer the skillet to a preheated oven at 350°F for about 10 minutes, or until the cheese is bubbly and golden.
