Preheat the oven to 375°F, grease a 9×13-inch baking dish, and set aside.
Boil the spaghetti noodles according to the package instructions until al dente. Drain and set aside.
In a large skillet set over medium-high heat, melt the butter. Sauté the garlic and bell pepper until tender.
Stir in the flour and Italian seasoning, and cook for 1 minute more.
Stir in the heavy whipping cream, broth, and milk. Bring the pan to a boil, reduce the heat and simmer until it begins to thicken.
Remove the pan from heat and stir in ½ cup of mozzarella until combined. Season with salt and pepper.
Stir the spaghetti, chicken, and diced tomatoes into the pan. Spread the mixture into the prepared baking dish.
Top with the remaining mozzarella cheese.
Bake for 15-20 minutes, until the edges are lightly browned and the cheese is bubbly.
