Step 1: Prep Chicken and Preheat Oven
Preheat your oven to 375°F (190°C). Slice the two chicken breasts horizontally into four thinner cutlets. Heat 1 tablespoon of olive oil in a large deep skillet over medium heat. Add the four chicken cutlets and cook them until golden brown and completely cooked through (about 7 to 10 minutes total). Remove the chicken to a clean cutting board to rest.

Step 2: Boil Pasta
While the chicken cooks, bring a large pot of water to a boil. Add 1/2 tablespoon of salt, then add the 8 oz of spaghetti. Boil the pasta until it is tender. Drain the cooked spaghetti in a colander.

Step 3: Start the Roux Base
In the same skillet that the chicken was cooked in, melt the 1/4 cup of butter over medium-high heat. Add the diced onion and minced garlic. Sauté for 3 to 4 minutes until the onion becomes translucent.

Step 4: Make and Thicken the Sauce
Sprinkle the 1/4 cup of all-purpose flour into the skillet and stir it continuously for 2 to 3 minutes until the flour turns a light golden color. Slowly pour the 2 cups of chicken broth into the skillet while stirring vigorously to prevent lumps. Once the sauce begins to thicken, add the 1/2 teaspoon of salt, 1/4 teaspoon of black pepper, 1/2 teaspoon of Italian seasoning, the heavy cream, 1/2 cup of the shredded cheddar, and 1/2 cup of the shredded mozzarella. Stir until the cheese is completely melted and the sauce is smooth.

Step 5: Assemble Casserole
Chop the rested chicken into 1/2 inch pieces. Turn the heat off the skillet. Add the drained diced tomatoes with green chiles, the chopped chicken pieces, and the cooked spaghetti to the pan. Mix everything until it is fully combined and coated in the sauce.

Step 6: Bake
Transfer the chicken spaghetti mixture to a 9×13 inch casserole dish. Top with the remaining 1/2 cup of cheddar cheese and 1/2 cup of mozzarella cheese. Bake the casserole for 25 minutes, or until it is bubbly around the edges and the cheese on top is fully melted and golden. Serve warm.